An analysis has been made of the operating principle of Boost and synchronous rectifier switch power supply circuit, and a design has been proposed for the main parameters and control circuit of Boost circuit, followed by the test of a low-power-consumption DC/DC Boost converter prototype based on LM5121, with 48 V its output voltage and 100 W its output power. Testing results substantiate the stable output voltage of the prototype, with an overall efficiency as high as 88%, thus proving it to be an economical and applicable switching power supply with low power consumption. Keywords ：low power consumption；Boost；switch power supply；synchronous rectification；LM5121