1. A resource allocation method for a MU-MIMO-OFDM system, the MU-MIMO-OFDM system has a plurality of users and sub-channels, and the method comprises: a plurality of preset assigned sub-channel numbers are assigned to the users according a plurality of capacity ratio constraints; each user sequentially selects one of the sub-channels according to a scheduling rule; determining one of the users who has a right of selecting the non-assigned sub-channels according to a plurality of assigned capacities and the capacity ratio constraints of the users, and then the user who has the right of selecting the non-assigned sub-channels selects one of the non-assigned sub-channels according to the scheduling rule, if the number of the sub-channels assigned to the user is equal to the preset assigned sub-channel number thereof, the user exits the competition of determining the right of selecting the non-assigned sub-channels in the step; determining if the number of the sub-channels assigned to the all of the users is equal to the summation of the preset assigned sub-channel numbers of all of the users; and determining one of the users who has the right of selecting the non-assigned sub-channels according to the assigned capacities and capacity ratio constraints of the users, and then the user who has the right of selecting the non-assigned sub-channels selects one of the non-assigned sub-channels according to the scheduling rule, the step is repeated until all of the sub-channels are assigned to the users.