In modern war weapons and inertial navigation systems, the fiber optic gyro (FOG) is required to start quickly. The effect of temperature on the scale factor error in the process of FOG startup is studied. The FOG startup time is closely related to the output performance of a superluminescent diode (SLD). The effect of the SLD output wavelength on the FOG scale factor error is studied, and the theoretical results are analyzed in simulation and verified experimentally. The results show that the change of SLD output wavelength caused by the temperature change seriously affect the scale factor error in the process of FOG startup. The larger the temperature change is, the larger the scale factor error in the process of FOG startup is. The maximum scale factor error is up to 0.048, 0.0144, 0.025 when the temperature is -20, 50, 70 ℃, and the startup time is up to 6, 7, 11 s respectively.