Embodiments of the present invention disclose a service transmission method and a first transmission device, and the method includes: mapping n client signals with a rate of t to ms/t load subareas of m lanes of FlexO frames, where a payload area of each of the m lanes of FlexO frames is divided into s/t load subareas, and each lane of FlexO frame is transmitted by using a FlexO lane with a transmission rate of s; configuring FlexO type information, timeslot overhead information, and signal mapping information for each lane of FlexO frame; and transmitting the m lanes of FlexO frames to a second transmission device by using m FlexO lanes with the transmission rate of s, where the second transmission device is configured to parse, according to the FlexO type information, the timeslot overhead information, and the signal mapping information, the client signals carried in the ms/t load subareas. According to the embodiments of the present invention, service transmission flexibility is improved.