The present invention discloses a data sending method and device, a data demodulation method and device, and a system. The data sending method includes: obtaining, by a sending device, cyclic prefix CP length determining information, where the CP length determining information includes at least one of data stream quantity information of data to be sent by the sending device, type information of the data, information about a distance between the sending device and a receiving device, width information of a beam transmitted by the sending device, type information of the beam, or power information of the beam; determining, by the sending device, a first CP length according to the CP length determining information; and sending, by the sending device, the data to the receiving device, where a CP of the first length is added to the data.