The invention relates to the field of mobile communication, in particular to a method and a system for charging a mobile terminal. By utilizing the characteristic that the credit rating of an afterpayment user is usually higher, establishing an alone fee counting platform in an afterpayment account at a network terminal and setting a chargeable limit when the afterpayment user needs to charge a prepaid user, the charging sum is recorded in the account of a user terminal which accepts charging after a network terminal detects that the identity of the afterpayment user is legal, the small alone fee counting account is available and the charging sum does not exceed the set limit, and charging fee owing information is recorded in the alone fee counting platform. On the basis that the existing network structure is not changed and the extra cost is not increased, the invention effectively solves the problem that the afterpayment user charges the prepaid user when the prepaid user has no balance or deficient balance and can not buy a charging card.