A channel state information transmission method, wherein the method comprises: sending, by a terminal device, a first reference signal through a first antenna port set to a network device, wherein the first antenna port set comprises n antenna ports, and n is a positive integer; receiving, by the terminal device, a second reference signal through a second antenna port set from the network device, wherein the second antenna port set comprises N antenna ports, N is a positive integer, n < N, and the first antenna port set is a proper subset of the second antenna port set; and sending, by the terminal device, channel state information to the network device, wherein content of the channel state information is associated with a rank indicator RI, and the channel state information and the RI are determined based on the second reference signal; wherein when RI ≤ X1, the content of the channel state information comprises a channel quality indicator CQI and/or the RI, and the channel state information does not comprise a precoding matrix indicator PMI; and when RI > X1, the content of the channel state information comprises the CQI and/or the RI, and the channel state information further comprises the PMI, wherein X1 is a positive integer less than or equal to N; or wherein when RI ≤ X2, the content of the channel state information comprises a CQI and/or the RI, and the channel state information further comprises a first PMI; and when RI > X2, the content of the channel state information comprises a CQI and/or the RI, and the channel state information further comprises a second PMI, wherein the first PMI and the second PMI are different elements in a first set, each element in the first set represents a different PMI type, the first set comprises at least two PMI types in a PMI indicating a channel eigenvector, a PMI indicating a channel vector, a PMI indicating a channel correlation matrix, a codebook-based PMI, an explicit feedback-based PMI, and an implicit feedback-based PMI, and X2 is a positive integer less than or equal to N. sending, by a terminal device, a first reference signal through a first antenna port set to a network device, wherein the first antenna port set comprises n antenna ports, and n is a positive integer; receiving, by the terminal device, a second reference signal through a second antenna port set from the network device, wherein the second antenna port set comprises N antenna ports, N is a positive integer, n < N, and the first antenna port set is a proper subset of the second antenna port set; and sending, by the terminal device, channel state information to the network device, wherein content of the channel state information is associated with a rank indicator RI, and the channel state information and the RI are determined based on the second reference signal; wherein when RI ≤ X1, the content of the channel state information comprises a channel quality indicator CQI and/or the RI, and the channel state information does not comprise a precoding matrix indicator PMI; and when RI > X1, the content of the channel state information comprises the CQI and/or the RI, and the channel state information further comprises the PMI, wherein X1 is a positive integer less than or equal to N; or wherein when RI ≤ X2, the content of the channel state information comprises a CQI and/or the RI, and the channel state information further comprises a first PMI; and when RI > X2, the content of the channel state information comprises a CQI and/or the RI, and the channel state information further comprises a second PMI, wherein the first PMI and the second PMI are different elements in a first set, each element in the first set represents a different PMI type, the first set comprises at least two PMI types in a PMI indicating a channel eigenvector, a PMI indicating a channel vector, a PMI indicating a channel correlation matrix, a codebook-based PMI, an explicit feedback-based PMI, and an implicit feedback-based PMI, and X2 is a positive integer less than or equal to N.