A channel state information, CSI sending method, comprising: determining, by a terminal device, a first rank used for a first channel and a second rank used for a second channel, wherein the first rank is indicated by a first rank indicator, the second rank is indicated by a second rank indicator, and a channel type of the first channel is different from a channel type of the second channel; determining, by the terminal device, a first precoding matrix in a first precoding matrix set, and determining a second precoding matrix in a second precoding matrix set, wherein the first precoding matrix set is different from the second precoding matrix set, a rank of the first precoding matrix is the first rank, the first precoding matrix is indicated by a first precoding matrix indicator, a rank of the second precoding matrix is the second rank, and the second precoding matrix is indicated by a second precoding matrix indicator; and sending, by the terminal device, the first rank indicator, the second rank indicator, the first precoding matrix indicator, and the second precoding matrix indicator to a base station; or sending, by the terminal device, the second rank indicator, the first precoding matrix indicator, and the second precoding matrix indicator to a base station, wherein the first rank indicator is a predefined value; wherein the first channel is a common channel and the second channel is a downlink traffic channel, wherein the common channel is a common control channel using which the base station sends control information to the terminal, and the downlink traffic channel is a downlink channel using which the base station sends downlink service data to the terminal device; wherein the first precoding matrix is W A and W A = W 1 A × W 2 A, wherein W A is an N t -row by R A -column matrix, N t is a quantity of antenna ports of a reference signal sent by the base station, R A is the first rank, N t is greater than or equal to R A, W 1 A is an N t -row by 2M-column matrix, W 2 A is a 2M-row by R A -column matrix, R A, M, and N t are positive integers, and M is greater than or equal to 2; and the second precoding matrix is W B and W B = W 1 B × W 2 B, wherein W B is an N t -row by R B -column matrix, R B is the second rank, W 1 B is an N t -row by 2K-column matrix, W 2 B is a 2K-row by R B -column matrix, and R B and K are positive integers; wherein K is less than or equal to M, and 2M columns in W 1 A comprise each column in W 1 B. determining, by a terminal device, a first rank used for a first channel and a second rank used for a second channel, wherein the first rank is indicated by a first rank indicator, the second rank is indicated by a second rank indicator, and a channel type of the first channel is different from a channel type of the second channel; determining, by the terminal device, a first precoding matrix in a first precoding matrix set, and determining a second precoding matrix in a second precoding matrix set, wherein the first precoding matrix set is different from the second precoding matrix set, a rank of the first precoding matrix is the first rank, the first precoding matrix is indicated by a first precoding matrix indicator, a rank of the second precoding matrix is the second rank, and the second precoding matrix is indicated by a second precoding matrix indicator; and sending, by the terminal device, the first rank indicator, the second rank indicator, the first precoding matrix indicator, and the second precoding matrix indicator to a base station; or sending, by the terminal device, the second rank indicator, the first precoding matrix indicator, and the second precoding matrix indicator to a base station, wherein the first rank indicator is a predefined value; wherein the first channel is a common channel and the second channel is a downlink traffic channel, wherein the common channel is a common control channel using which the base station sends control information to the terminal, and the downlink traffic channel is a downlink channel using which the base station sends downlink service data to the terminal device; wherein the first precoding matrix is W A and W A = W 1 A × W 2 A, wherein W A is an N t -row by R A -column matrix, N t is a quantity of antenna ports of a reference signal sent by the base station, R A is the first rank, N t is greater than or equal to R A, W 1 A is an N t -row by 2M-column matrix, W 2 A is a 2M-row by R A -column matrix, R A, M, and N t are positive integers, and M is greater than or equal to 2; and the second precoding matrix is W B and W B = W 1 B × W 2 B, wherein W B is an N t -row by R B -column matrix, R B is the second rank, W 1 B is an N t -row by 2K-column matrix, W 2 B is a 2K-row by R B -column matrix, and R B and K are positive integers; wherein K is less than or equal to M, and 2M columns in W 1 A comprise each column in W 1 B.