The research showed the simulation of position error when assembling a reflective mirror of parabolic trough concentrator. The shape of a reflective mirror is like a parabolic cylinder model, relying on the back of the four-point mounted on a special setup, making it unable to move. Therefore, it is of great importance of the machining precision of special bracket. We need to analyze the influence of reflective mirror‘s intercept factor in order to guide the processing precision. It is assumed that each reflective mirror is rigid, we have calculated the intercept factor of reflector with mounting points’ random error of different standard deviation, comparing the simulating results with TRACEPRO. As a sequence, we can approve the feasibility of the algorithm, and give the effect of different random errors on the light-gathering efficiency. On the basis, we provide the machining accuracy of bracket. The simulation results show that when the mounting points’ standard deviation of position error is less than 0.5 mm, the intercept factor of receiver has reached upwards of 92% with 60 mm diameter for receiver, which can satisfy the design requirements.