Four-quadrant detector (QD) is used for detector of extract track in free space optical communication, and influences tracking precision. The method of the variable step-size is given, and the influences of the facular size to the dynamic range and the detection sensitivity are analyzed. As the simulation precision is satisfied, simulation time is shortened. On the condition that the facular energy obeys the Gauss distribution, the influences of the background light, responsibility and dead-area to the dynamic range and the detection sensitivity are analyzed, especially the influence of the non-uniformity background light is analyzed. The result indicates that the dynamic range of facular position detection is increased, and detection sensitivity is reduced with the increase of the facular size. On the condition of the same facular size, the detection sensitivity as to obey Airy distribution is higher than Gauss distribution, the influence of non-uniformity to the detector is larger than uniformity.