Assigning partial charges

I want to simulate ionic liquid and want to assign partial charges to each atom of cation, is anyone here know method for this?

How to construct partial charges is usually included in the description of the force field you use. Some force fields use an increment system, others prescribe some quantum chemical calculations followed by a fitting procedure.

You have to look up the details in the publication describing the force field that you want to apply. If the force field does not provide this information (some very old force fields are like that), then this is an indication that it was not developed for simulating condensed matter, but only to represent molecular structure (i.e. as a cheap approximation to quantum chemical calculations).