Optical time domain reflectometer plays a very important role in the test of optical fiber system, how to improve its resolution and dynamic range has become a difficulty.In order to make the designed demodu-lation system to achieve the requirements for measuring accuracy and dynamic range, the Xilinx xc3s400and USB singlechip were adopted as the main core device, the functions such as A/D sampling and communications were completed respectively, A/D takes12150Mbps fast-sampling chips to accomplish the quantization of the measurement.Communications are realized in USB communication mode, in order to realize data collection and data transfer between PC.A data transfer rate reached over30M.In developing visual graphic interface PC complete data processing and results.