In practical application, plane measuring instrument is mainly used to test flatness of workpiece in micro level, so that people can check if the workpiece is qualified or not. High resolution and high stabilization are two key technologies. The method of resolution extension is presented for improving the resolution of workpiece,and a signal source with high stabilization, which is designed by microcontroller and Complex Programmable Logic Device(CPLD), improves the stabilization of the system. The paper propose work principle and test step, the design of hardware,and introduces the measure for improving precision. The system is consisted of a position transducer, a signal source with high stabilization, a microcontroller and a signal processing unit. It has many advantages such as high resolution, high stabilization, low cost and simple operation, It also has been formally put to use in Chongqing Actaris Measurement Instrument Co.Ltd. now.