Previous studies have shown that there exists a large nonreciprocal phase error in the single-mode fiber coil of a depolarized interference fiber optic gyroscope (De-IFOG) under time-varying temperature and magnetic field, which evidently weakens its environmental adaptability. A real-time software compensation method considering the coupling effect of time-varying temperature and magnetic field is first proposed and verified experimentally. After compensation, the error of De-IFOG decreases from 50  deg  /  h to 6  deg  /  h when the magnitude of the magnetic field is 10 G and temperature uniformly increases from 10°C to 60°C with time-varying rate of 36  °  C  /  h. The compensation method is of great significance in De-IFOG engineering applications.