The paper introduces the configuration, function, principles and main technical index of the satellite-based direct sequence spread spectrum/phase modulation(DS/PM)dual-mode transponder. An accurate demodulation phase method is presented in PM mode. Obviously the processing precision in PM mode is improved and the capturing bandwidth widened by this means. Some key means are also developed such as capturing the initial phase of Pseudo Noise Code in spread-spectrum mode in the environment of dynamic changes and low SNR, and synchronizing the carrier wave with large Doppler frequency shift. All these ideas and methods are very valuable in developing space technology.