This application relates to the field of satellite communication technologies, and discloses an outbound data transmission method in a BeiDou communication system, so that a terminal can add MCS information to an inbound first user frame, and send the inbound first user frame to a BeiDou network device, to indicate the BeiDou network device to encode and modulate, based on the MCS information recommended by the terminal, service data to be sent to the terminal. In this way, the terminal can feed back an outbound MCS to the BeiDou network device without additional signaling overheads, to improve channel utilization.