A method for measuring channel state information, applied to a communications system comprising a network device (20, 30) and a terminal device (10, 40), wherein a system frequency resource used by the communications system is divided into X subbands, and the method comprises: determining, by the terminal device (10, 40), a first frequency resource, wherein Z subbands comprised in the first frequency resource are determined by the network device (20, 30) in a first time unit in Y subbands comprised in a second frequency resource, the second frequency resource is a downlink frequency resource allocated by the network device (20, 30) to the terminal device (10, 40), the Y subbands are comprised in the X subbands, and the second frequency resource corresponds to one piece of wideband channel state information, CSI, wherein X≥Y≥2, Y>Z≥1, and X, Y, and Z are all integers; determining, by the terminal device (10, 40), the wideband CSI based on the first frequency resource; and sending, by the terminal device (10, 40), the wideband CSI to the network device (20, 30) in a second time unit, wherein the method further comprises: determining, by the terminal device (10, 40), K1 pieces of first CSI and/or K2 pieces of second CSI based on the first frequency resource, wherein K1≥1 and K2≥1, the first CSI is CSI determined by the terminal device (10, 40) based on Z1 subbands in the Z subbands, Z>Z1≥1, the second CSI is CSI determined by the terminal device (10, 40) based on some downlink frequency resources comprised in one of the Z subbands, and K1, K2, and Z1 are all integers; and sending, by the terminal device (10, 40), the K1 pieces of first CSI and/or the K2 pieces of second CSI to the network device (20, 30) in the second time unit. determining, by the terminal device (10, 40), a first frequency resource, wherein Z subbands comprised in the first frequency resource are determined by the network device (20, 30) in a first time unit in Y subbands comprised in a second frequency resource, the second frequency resource is a downlink frequency resource allocated by the network device (20, 30) to the terminal device (10, 40), the Y subbands are comprised in the X subbands, and the second frequency resource corresponds to one piece of wideband channel state information, CSI, wherein X≥Y≥2, Y>Z≥1, and X, Y, and Z are all integers; determining, by the terminal device (10, 40), the wideband CSI based on the first frequency resource; and sending, by the terminal device (10, 40), the wideband CSI to the network device (20, 30) in a second time unit, wherein the method further comprises: determining, by the terminal device (10, 40), K1 pieces of first CSI and/or K2 pieces of second CSI based on the first frequency resource, wherein K1≥1 and K2≥1, the first CSI is CSI determined by the terminal device (10, 40) based on Z1 subbands in the Z subbands, Z>Z1≥1, the second CSI is CSI determined by the terminal device (10, 40) based on some downlink frequency resources comprised in one of the Z subbands, and K1, K2, and Z1 are all integers; and sending, by the terminal device (10, 40), the K1 pieces of first CSI and/or the K2 pieces of second CSI to the network device (20, 30) in the second time unit. determining, by the terminal device (10, 40), K1 pieces of first CSI and/or K2 pieces of second CSI based on the first frequency resource, wherein K1≥1 and K2≥1, the first CSI is CSI determined by the terminal device (10, 40) based on Z1 subbands in the Z subbands, Z>Z1≥1, the second CSI is CSI determined by the terminal device (10, 40) based on some downlink frequency resources comprised in one of the Z subbands, and K1, K2, and Z1 are all integers; and sending, by the terminal device (10, 40), the K1 pieces of first CSI and/or the K2 pieces of second CSI to the network device (20, 30) in the second time unit.