This paper investigates the distance measurements performance of ranging by POA(phase of arrive) and implements a small underground parking lot positioning system based on POA. AT86RF233 uses phase shift measurements of different carrier waves to calculate the distance. This paper designs the variable amplitudelimiting and average filtering algorithm to improve realtime and smoothness performance of ranging results. Then implements the centroid localization algorithm based on weighted minimum mean square error which improves the centroid selection method of centroid localization algorithm in 3anchors indoor position system. The field experiments’ results verifies the effectiveness of the algorithms. the tracking accuracy of the indoor positioning system reaches submeter level and it can be used for vehicle and personnel tracking in small indoor parking lots.