Abstract: Theory of coherent demodulation based on method of square counterchange is introduced in this article.The structure of circuit for carrier recovery is perfected in engineering, and the problem of error-lock for frequency-shift carrier is eliminated.Costas Loop is put forward to achieve coherent carrier from 2DPSK signal in this article.The 180° phase's inexplicit problem which exist in carrier recovery is resolved.It improves the resisting noise property of communication system.When carrier frequency is higher, Costas loop is easy to implement carrier synchronization.Because the operating frequency of Costas loop is lower than square PLL.We scheme out emulational circuit diagram for carrier distilling with Costas loop by using simulink and process corresponding emulational experiment.