In an aircraft salt fatigue test, the follow-up loading technology is used to apply the follow-up loading when the open angle of aircraft salts changes. In allusion to the errors between the rotation angles of 5 swing-arm follow-up loading devices and the open angle of slats, the geometric mathematical models of devices are studied in this paper. Combined with the test load spectrum waveform, the relationship with the actuator displacements and the reason of the angle errors are confirmed. According to analysis of angle errors influenced by parameters of the slat follow-up loading devices, an improved method of slat follow-up loading technology based on angle errors is proposed. This method which is proved in the aircraft salt fatigue test can effectively reduce the angle errors and guide follow-up loading devices designing and load spectrum choosing.