Thanks so much for your reply, now, everything is OK, it was the problem of path setting (fftw and mpich2)
In order for facilitating other lammps user installing lammps in linux system, I paste my steps on installing lammps and Makefile.g++ 
ps:
First, set a folder under your account for installing lammps and its library.
cd /home/xie
mkdir lmp
Second, download packages of lammps, fftw, mpich, and jpeg into folder temp which I have already download and unpack
mkdir temp
cd temp(assuming that it contains four packages of the software we need to install)
third, install the packages in folder temp and set its library in folder lmp
unpack this three packages and install them
tar -zvxf jpegsrc.v8b.tar.gz
tar -zvxf fftw-2.1.5.tar.gz
tar-zvxf mpich-1.2.7p1
tar -zvxf lammps.tar.gz
? Install JPEG LIB (the latest version of lammps need JPEG lib)
cd jpeg-8b
./configure --prefix=/home/xie/lmp/jpeg (arrange the PATH of JPEG)
make
make install
? Install fftw
cd ..
cd fftw-2.1.5
./configure --prefix=/home/xie/lmp/fftw2 --enable-float
make
make install
? Install mpich
cd ..
cd mpich2-1.4.tar
./configure --prefix=/home/xie/lmp/mpich2
make
make install
Forth, after it has been done, there should be three folders arising in folder lmp (jpeg, fftw, mpich); then we can go to the next step------install lammps
cd ..
mv lammps-1Jul11 /home/xie/lmp??（copy lammps-19Jul11 to lmp）
cd lammps-1Jul11/src
make g++
ps:
the modified makefile.g++ (the modified parts with red color)
# g++ = RedHat Linux box, g++4, gfortran, MPICH2, FFTW
SHELL = /bin/sh
# ---------------------------------------------------------------------
# compiler/linker settings
# specify flags and libraries needed for your compiler
CC = g++
CCFLAGS = -g -O # -Wunused
DEPFLAGS = -M
LINK = g++
LINKFLAGS = -g -O
LIB =
ARCHIVE = ar
ARFLAGS = -rc
SIZE = size
# ---------------------------------------------------------------------
# LAMMPS-specific settings
# specify settings for LAMMPS features you will use
# LAMMPS ifdef options, see doc/Section_start.html
LMP_INC = -DLAMMPS_GZIP -DLAMMPS_JPEG
# MPI library, can be src/STUBS dummy lib
# INC = path for mpi.h, MPI compiler settings
# PATH = path for MPI library
# LIB = name of MPI library
MPI_INC = -DMPICH_SKIP_MPICXX -I/home/xie/lmp/mpich2/include
MPI_PATH = -L/home/xie/lmp/mpich2/lib
MPI_LIB = -lmpich -lfmpich -lmpl -lpthread
# FFT library, can be -DFFT_NONE if not using PPPM from KSPACE package
# INC = -DFFT_FFTW, -DFFT_INTEL, -DFFT_NONE, etc, FFT compiler settings
# PATH = path for FFT library
# LIB = name of FFT library
FFT_INC = -DFFT_FFTW -I/home/xie/lmp/fftw2/include
FFT_PATH = -L/home/xie/lmp/fftw2/lib
FFT_LIB = -lfftw
# JPEG library, only needed if -DLAMMPS_JPEG listed with LMP_INC
# INC = path for jpeglib.h
# PATH = path for JPEG library
# LIB = name of JPEG library
JPG_INC = -I/home/xie/lmp/jpeg/include
JPG_PATH = -L/home/xie/lmp/jpeg/lib
JPG_LIB = /home/xie/lmp/jpeg/lib/libjpeg.a
#/usr/local/lib/libjpeg.a
# additional system settings needed by LAMMPS package libraries
# these settings are IGNORED if the corresponding LAMMPS package
# (e.g. gpu, meam) is NOT included in the LAMMPS build
# SYSINC = settings to compile with
# SYSLIB = libraries to link with
# SYSPATH = paths to libraries
gpu_SYSINC =
meam_SYSINC =
reax_SYSINC =
user-atc_SYSINC =
user-awpmd_SYSINC =
gpu_SYSLIB = -lcudart -lcuda
meam_SYSLIB = -lgfortran
reax_SYSLIB = -lgfortran
user-atc_SYSLIB = -lblas -llapack
user-awpmd_SYSLIB = -lblas -llapack
gpu_SYSPATH = -L/usr/local/cuda/lib64
meam_SYSPATH =
reax_SYSPATH =
user-atc_SYSPATH =
user-awpmd_SYSPATH =
# ---------------------------------------------------------------------
# build rules and dependencies
# no need to edit this section
include Makefile.package
EXTRA_INC = \(LMP\_INC\) (PKG_INC) \(MPI\_INC\) (FFT_INC) \(JPG\_INC\) (PKG_SYSINC)
EXTRA_PATH = \(PKG\_PATH\) (MPI_PATH) \(FFT\_PATH\) (JPG_PATH) \(PKG\_SYSPATH\)
EXTRA\_LIB = (PKG_LIB) \(MPI\_LIB\) (FFT_LIB) \(JPG\_LIB\) (PKG_SYSLIB)
# Link target
\(EXE\): (OBJ)
\(LINK\) (LINKFLAGS) \(EXTRA\_PATH\) (OBJ) \(EXTRA\_LIB\) (LIB) -o \(EXE\)
(SIZE) $(EXE)
# Library target
lib: \(OBJ\)
(ARCHIVE) \(ARFLAGS\) (EXE) $(OBJ)
# Compilation rules
\.o:.cpp
\(CC\) (CCFLAGS) \(EXTRA\_INC\) \-c <
\.d:.cpp
\(CC\) (CCFLAGS) \(EXTRA\_INC\) (DEPFLAGS) < > @
# Individual dependencies
DEPENDS = \(OBJ:\.o=\.d\)
include (DEPENDS)