The present invention provides an asynchronous data multiplexing method and device. The method includes the following steps: acquiring status flag, serial number, and data information of each branch data; performing asynchronous data buffering processing on the data information of each branch data according to preset channel transmission bit-serial number to obtain buffered data of each branch; then determining each branch data according to the status flag, if the status flag is valid identification information, a flag bit corresponding to a frame header is set according to the valid identification information, and the buffered data of the current branch data is added to a corresponding position of the data part by bit; and if the status flag is invalid identification information, the flag bit corresponding to the frame header is kept unchanged.