Abstract: Various methods of indoor positioning systems are analyzed and compared in this paper. In order to achieve higher precision of positioning, the Time Difference of Arrival (TDOA) method is used for positioning. An indoor positioning prototype based on Wi-Fi is designed and it's implemented by USRP 2952R Software Defined Radio (SDR) platform from NI. The whole system includes the receiving of Wi-Fi protocol, time difference calculation and position computation and display. The test results show that it could reach an accuracy and precision of 0.3 m, and the refresh rate for each point is 1 s, which could meet the requirement of indoor positioning system.