An embodiment of the invention provides a method and a device for generating an optical signal based on geometric-probability shaping. The method comprises the following steps: acquiring original datato be sent; determining, according to sequence numbers of each constellation subset, a subset serial number sequence obeying the optimal distribution corresponding to the original data, and determining a binary sequence of the subset serial number sequence obeying the optimal distribution corresponding to the original data according to binary labels of each of the constellation subsets, wherein the constellation subsets are obtained by performing subset division on the multi-mode circular constellation after performing geometric forming on square constellation diagrams of the preset number ofconstellation points to obtain a multi-mode circular constellation; systematically coding the binary sequence to obtain the encoded information; sequentially selecting a first preset number of information bits and a second preset number of redundant bits for the encoded information, and mapping the above bits to the multi-mode circular constellation to obtain an optical signal to be transmitted.This embodiment enables the transmitted light signal to approximate the Shannon channel capacity.