An ambient WiFi energy harvesting system is presented in this paper,with operating frequency from 2.4 GHz to 2.485 GHz.A voltage multiple rectifier is used in this design,and a broadband matching network for the circuit from antenna to rectifier is proposed to improve the efficiency of energy harvesting.S₁₁ of the proposed broadband matching circuit is less than -10 dB within the WiFi working frequency range.The harvesting signal can be increased by 4 times through the rectifier circuit,thus effectively improve the RF-DC conversion efficiency.Experiment results show that,with an input power of -10 dBm,the design achieves 40% of RF-DC conversion efficiency and the super capacitor harvests voltage of 257 mV in 30 minutes.