Abstract: Position sensing detector (PSD) is an optics sensor that can detect the center of the incident light. It has the advantages of fast response, high position resolution and simple circuit, these characteristics make it a wide range of application in precision measurement such as angle and position detection. A optical experiment on a optic bench A823 was chose for data acquisition under the DOS operating system and Borland C++ builder, C program was written for a real-time transformation between analog voltage and digital voltage, then position coordinate was calculated and the error was analyzed. 2L10_SU65_SPC02 PSD used in this experiment is from Sitek corporation in Sweden, through the error analysis the main conclusions include fringe effect of PSD, the main influence about the position error are natural daylight and fluorescent light. If eliminate influence factor precision will improve a lot.