In order to make up for the disadvantages of traditional CCD method and monitor particle′s position quickly and high precisely, back-focal-plane (BFP) method is used to detect the particle′s position in an optical trap. The basic principle of BFP method for position detection is explained, then the experimental system is built and the target that detect the particle′s position quickly and high precisely is achieved. The experimental result shows that the detection accuracy reaches 80 nm and the response frequency reaches 800 Hz，so it can be used to detect the micro particle′s position in an optical trap.