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 comprising: (a) assigning, by a device, a plurality of preset assigned sub-channel numbers to the users according to a plurality of capacity ratio constraints, wherein the capacity ratio constraints are computed according to a number of the users, whether the n-th sub-channel is occupied by a k-th user, and all eigenmodes of the n-th sub-channel when the n-th sub-channel is assigned to the k-th user; (b) each user sequentially selecting one of the sub-channels according to a scheduling rule; (c) determining one of the users who has a right of selecting 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 a number of the sub-channels assigned to the user is equal to the preset assigned sub-channel number thereof, the user exits a competition of determining the right of selecting the non-assigned sub-channels in the step (c); (d) determining if a number of the sub-channels assigned to the all of the users is equal to a summation of the preset assigned sub-channel numbers of all of the users; (e) 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; and (f) repeating the step (e) until all of the sub-channels are assigned to the users.