Abstract: Aim To study the design and application of X.21-G.703 converter Meth- ods principle of the line signal coding was analyzed by means of the power spec- trum of the baseband signal and the G.703,X.21 protocols were analyzed also.ResultsIt suggested a useful implementation of coding and decoding scheme and presented its principle of circuit design and application Conclusion The system works steadily and completes the specified function well.