Abstract: Various kinds of positioning methods and time interval measurement methods are analyzed and compared in this paper. The Time Difference Of Arrival (TDOA) method is used for positioning aimed at the indoor positioning system. The paper has designed a kind of delay line interpolation type Time to Digital Converter(TDC) circuit based on FPGA and the Xilinx Spartan-6 series FPGA is used. The whole TDC system includes meticulous time measurement module, logic control module, rough counter module and data display module. The research status of the indoor positioning technique and TDC is firstly introduced in this paper and the system framework of TDC and the principle and design of each parts are described. This paper is focused on the design of meticulous time measurement module and finally provides the simulation results of some parts and the measured results of TDC system. The measuring accuracy is less than 200 ps, which can meet the demand of the indoor positioning system.