The present invention provides a method and a coder for jointly coding Pre-coding Matrix Index #1 (W1) and Rank Index (RI) for channel state information feedback of a double codebook precoding frame. In the present invention, it is determined whether Rank Index RI is more than 2; if the RI is no more than 2, a first half branch of a coding binary tree is used to represent joint coding of the RI and W1 when RI=1 and RI=2; if the RI is more than 2, a second half branch of the coding binary tree is used to represent remaining separate codings and/or joint codings when the RI is more than 2; and processing results of the above coding steps are output. The present invention may be applied to an equal-length coding or an unequal-length coding. The present invention has advantages such as easy implementation, lower signaling overhead etc., and may be suitable for an LTE-A/4G cellular communication system and a future 5G cellular communication system.