In target tracking applications, active ultrasonic sensors can provide satisfactory distance estimations, but also suffer from inter-sensor-interference when they are not well scheduled. In this paper, we propose a distributed saturation degree based algorithm (DSDA), which assigns the TDMA slot in wireless sensor networks distributively in order to avoid the interference. By adopting a graph coloring technique, Saturation Degree Heuristic, this new algorithm can provide near-optimal slot number in a totally distributed way. Simulation results demonstrate the efficiency of DSDA in terms of slot number, system scalability, tracking accuracy and energy consumption.