The invention discloses a radar work channel acquisition system based on two-stage digital down conversion. A radio frequency direct sampling module obtains radar echo digital signals including n channels, and a digital orthogonal frequency mixing module obtains baseband digital signals including n channels; a half-band filter module obtains the baseband digital signals including n channels after the first stage of digital down conversion, and a data transmitting interface module obtains frame data; a radar parameter information module is used for providing parameter information of the current radar work channel; the corresponding frequency mixing control word is outputted of a channel mapping table module, and a data receiving interface module acquires the baseband digital signals including n channels after the first stage of digital down conversion and corresponding to the frame data; and the frequency-variable digital orthogonal frequency mixing module obtains the baseband digital signals of which the work channel is at zero frequency after digital orthogonal frequency mixing, and a multiphase extraction filter module obtains the baseband digital signals only including the work channel after the second stage of digital down conversion and then outputs the baseband digital signals to a data transmitting optical fiber module to be outputted.