A channel state information CSI reporting method, comprising: receiving, by a first communications apparatus, reference signals from a second communications apparatus; and performing, by the first communications apparatus, channel measurement based on the reference signals, and sending first CSI to the second communications apparatus in a time unit whose identifier is m, wherein the first CSI is used to indicate a channel state in a first reference time unit and a channel state in a second reference time unit, wherein an identifier of the first reference time unit is m-n 1, an identifier of the second reference time unit is m-n 1 -n 2, m, n 1, and n 2 are integers, and n 2 is not equal to 0, wherein the first CSI comprises a first precoding matrix indicator PMI, a second PMI, and a third PMI, wherein the first PMI is used to indicate a matrix W 1 corresponding to the first reference time unit and the second reference time unit, the second PMI is used to indicate a matrix W 2 1 corresponding to the first reference time unit, the third PMI is used to indicate a matrix W 2 2 corresponding to the second reference time unit, a precoding matrix corresponding to the first reference time unit satisfies W 1 = W 1 × W 2 1, and a precoding matrix corresponding to the second reference time unit satisfies W 2 = W 1 × W 2 2, wherein the third PMI is used to indicate a relative amplitude value Δ p x, y, z, and the relative amplitude value Δ p x, y, z is specifically used to represent a relative value between an amplitude coefficient p x, y, z 2 in an ( x × l + z) th row and a y th column in W 2 2 and an amplitude coefficient p x, y, z 1 at a corresponding location in W 2 1; and/or the third PMI is used to indicate a relative phase value Δ c x, y, z, and the relative phase value Δ c x, y, z is specifically used to represent a relative value between a phase coefficient c x, y, z 2 in the ( x × l + z) th row and the y th column in W 2 2 and a phase coefficient c x, y, z 1 at a corresponding location in W 2 1, wherein the matrices W 2 1 and W 2 2 each are a matrix of 2l rows and N columns, both l and N are positive integers, x and z represent row numbers of W 2 1 and W 2 2, y represents column numbers of W 2 1 and W 2 2, x ∈ {0,1}, z ∈ {1,2,···, l}, and y ∈ {1,2,···, N}. receiving, by a first communications apparatus, reference signals from a second communications apparatus; and performing, by the first communications apparatus, channel measurement based on the reference signals, and sending first CSI to the second communications apparatus in a time unit whose identifier is m, wherein the first CSI is used to indicate a channel state in a first reference time unit and a channel state in a second reference time unit, wherein an identifier of the first reference time unit is m-n 1, an identifier of the second reference time unit is m-n 1 -n 2, m, n 1, and n 2 are integers, and n 2 is not equal to 0, wherein the first CSI comprises a first precoding matrix indicator PMI, a second PMI, and a third PMI, wherein the first PMI is used to indicate a matrix W 1 corresponding to the first reference time unit and the second reference time unit, the second PMI is used to indicate a matrix W 2 1 corresponding to the first reference time unit, the third PMI is used to indicate a matrix W 2 2 corresponding to the second reference time unit, a precoding matrix corresponding to the first reference time unit satisfies W 1 = W 1 × W 2 1, and a precoding matrix corresponding to the second reference time unit satisfies W 2 = W 1 × W 2 2, wherein the third PMI is used to indicate a relative amplitude value Δ p x, y, z, and the relative amplitude value Δ p x, y, z is specifically used to represent a relative value between an amplitude coefficient p x, y, z 2 in an ( x × l + z) th row and a y th column in W 2 2 and an amplitude coefficient p x, y, z 1 at a corresponding location in W 2 1; and/or the third PMI is used to indicate a relative phase value Δ c x, y, z, and the relative phase value Δ c x, y, z is specifically used to represent a relative value between a phase coefficient c x, y, z 2 in the ( x × l + z) th row and the y th column in W 2 2 and a phase coefficient c x, y, z 1 at a corresponding location in W 2 1, wherein the matrices W 2 1 and W 2 2 each are a matrix of 2l rows and N columns, both l and N are positive integers, x and z represent row numbers of W 2 1 and W 2 2, y represents column numbers of W 2 1 and W 2 2, x ∈ {0,1}, z ∈ {1,2,···, l}, and y ∈ {1,2,···, N}.