A data retransmission method, a relay station, a base station, and a communication system are provided. The data retransmission method includes: receiving a reception report sent by a base station and source data frames of a current transmission period that are sent by a user equipment through a subchannel, where the reception report carries sequence numbers of source data frames of a previous transmission period and the source data frames are not correctly received by the base station; obtaining according to the reception report and pre-stored source data frames of the previous transmission period, a set C1 of source data frames of the previous transmission period that need to be retransmitted; performing joint encoding for the source data frames in the set C1 and the source data frames of the current transmission period; and sending forwarded data frames generated by joint encoding to the base station through the subchannel.