This application provides a data transmission method, a radio frequency unit, a baseband unit, and a communication system. The baseband unit includes: a multi-wavelength integrated light source, configured to generate a first optical carrier and a second optical carrier that have different wavelengths; a first modulator, configured to modulate downlink data sent to the first radio frequency unit to the first optical carrier, to obtain a first downlink optical signal; a second modulator, configured to modulate downlink data sent to the second radio frequency unit to the second optical carrier, to obtain a second downlink optical signal; and a multiplexer/demultiplexer, configured to multiplex the first downlink optical signal and the second downlink optical signal into a downlink combined optical signal and send the downlink combined optical signal. In this solution, each optical module in the baseband unit is deconstructed, and a multi-wavelength integrated light source that can provide optical carriers having a plurality of different wavelengths is disposed in the baseband unit, so as to reduce a quantity of light sources in the baseband unit, and help a plurality of multi-channel modulators in the baseband unit be integrated on a monolithic, to save an area occupied by separate encapsulation of each optical module.