Abstract: This paper introduces a simple tracking system of measuring work function with good quality. The system includes a vacuum diode, with a low-energy electron gun as the cathode and a sample as the anode, and a feedback circuit of controlling the sample current at a constant value within retarding field. The anode voltage is monitored by X-Y recorder, Thus, the variation of the work function of the sample can be obtained. This system is applied to the study of surface states of YBa_2Cu_3O_(7-x) and the oxygen-adsorbing characteristic of the sample is obtained.