The invention belongs to the field of solar heat power generation, and relates to a tracking precision detecting method of a sunlight tracking system. The method includes the following steps that a circular plane mirror is installed on the illuminated side of a dish frame of the sunlight tracking system, the dish frame is made to face sunlight, sunlight is reflected by the plane mirror to a heat collector opening plane of the sunlight tracking system to form a circular light spot, the outer contour of the circular light spot is drawn on the heat collector opening plane, and a ruler is drawn in the circumferential direction of the outer contour with the outer contour as the reference circle, wherein the diameter of the circular light spot is the same as the diameter of the plane mirror; adjusting the dish frame before the sun is tracked by the sunlight tracking system so that the outer contour of the reflected light spot of the plane mirror can coincide with the reference circle on the heat collector opening plane; measuring the deflection distance between the light spot of the reflected light of the plane mirror and the reference circle through the ruler when tracking deflection happens, calculating the deflection amount of the sunlight tracking system through the deflection distance, and making the deflection amount represent the tracking precision of the sunlight tracking system.