The present disclosure provides a data transmission method including: receiving a first-mode optical signal from a first port corresponding to a first port number; converting, according to a correspondence between the first port number and a first mode group number, the received first-mode optical signal into a second-mode optical signal carried in a first mode group identified by the first mode group number, where the second-mode optical signal carried in the first mode group identified by the first mode group number includes an optical signal in one or more modes; and outputting the second-mode optical signal obtained by means of conversion.