This thsis described the function and the logic circuit structure of the CINRAD/SA encoder unit ,and analyzed the concrete function of resolve excitation signal generator, rotary transformers, the Resolve to the Digit Converter and the Programmable Logic Devices. Then it was analyzed deeply that how the encoder unit used the the synthesis and correction program control strategy of coarse codes and fine codes to achieve high precision radar antenna control. Finally, this paper analyzed the practical application of the encoder unit in the CINRAD/SA project which can prove that the performance target was achieved. References about maintenance and research of CINRAD/SA servo system can be provided to weather radar technology support staffs at all levels.