The present disclosure discloses a channel state information feedback method, a base station, a terminal device, and a system. The method includes: receiving, by a terminal device, a reference signal sent by a base station; determining, by the terminal device, channel state information based on the reference signal; and sending, by the terminal device, the channel state information to the base station, so that the base station determines a precoding matrix F. The precoding matrix F=C1C2W, where C1 is a first-level precoding matrix, C2 is a second-level precoding matrix, and W is a third-level precoding matrix. Channel state information corresponding to C1 includes beam information, channel state information corresponding to C2 includes long-term wideband channel information, and channel state information corresponding to W includes instantaneous channel information. This patent application can reduce total pilot and CSI feedback overheads, thereby improving a system throughput.