A space optical communication system with intensity modulation direct detection based on four quadrant detector tracking and communication multiplexing has been reported, which uses the dual optical wedge driven by ultrasonic motor as the beam deflection execution unit to form a closed-loop system for beam position tracking. The rotation period of the driving motor is 15 ms, and the position resolution is 0.83 μrad. Through theoretical analysis and experimental verification, the position closed-loop tracking -3 dB bandwidth of this system is about 4 Hz. When the position detection error is less than 10%, that is, the beam detection accuracy is less than 12 μrad, and the corresponding detection sensitivity is -45.2 dBm. At a communication rate of 10 Mbit/s and without signal encoding, the corresponding communication sensitivity is -44 dBm when the bit error rate is 1×10^-3. It has been verified that it is feasible to use the four-quadrant detector as a tracking and communication multiplexing detector, which can be applied to small and lightweight interstellar laser communication terminals.