Reference signal sequence mapping and de-mapping methods, and an apparatus are provided, to resolve a problem in a conventional technology that time-domain overheads of a reference signal of a single-carrier waveform are relatively large. In this application, a sending device maps a reference signal sequence to a modulation symbol set, to obtain a multiplexing set, where the multiplexing set occupies one single-carrier time domain symbol. Compared with a solution in which a reference signal sequence occupies one single-carrier time domain symbol, the solution in this application reduces time-domain overheads of the reference signal. In addition, to correctly perform channel estimation or channel sounding for the reference signal, different reference signal sequences meet conditions such as orthogonality or conjugate orthogonality, and locations of the reference signal sequences in different multiplexing sets further meet corresponding conditions, and so on.