In view of the test requirement of 400 Gb/s optical module in research and development and production, a low-cost 4-level pulse amplitude modulation(PAM4) signal bit error ratio tester(BERT) design scheme is proposed. Firstly, the overall scheme and implementation principle of BERT are introduced, and the requirement of 400 Gb/s optical module test for BERT and the characteristics of digital signal processing(DSP) chip inside the optical module are analyzed. Secondly, according to different application scenarios, two hardware implementation forms and software design points of BERT are detailed. Finally, the BERT is tested in practice, and the results of PAM4 electrical signal and optical signal are given, and the results show that the BERT can meet the needs of practical application.