The simulation of the size of the target laser facular is a key process of the Hardware-in-the-loop (HWIL) simulation of laser guided weapons. Through the analysis of the principle of laser guidance seeker, the model for position and size of laser facular in laboratory is built. According to the principle of comparability, the telescope system is used to control the movement of the size of laser facular, and the real target laser facular sought by the guidance seeker is simulated. Based on the mathematical model, the simulation experiments were carried out. The experimental results prove that the mathematic models are right and the simulation way of the size of the target laser facular is realizable, which resolves the key problem of HWIL simulation of laser guided weapon.