The invention provides a method for improving vehicle light modulation signal stability. The method comprises steps that whether a voltage of a vehicle storage cell surpasses a preset voltage scope is determined, if yes, light modulation signal sampling is shielded, if not, the process enters a next step; whether a light modulation signal sampling value is stable is determined, if yes, the process enters a next step, if not, the light modulation signal sampling value is discarded, and re-sampling is carried out; whether a light modulation decision voltage needs calibration is determined, if yes, fluctuation compensation correction for the light modulation decision voltage is carried out, if not, the process enters a next step; the light modulation decision voltage is compared with a preset light modulation reference voltage to acquire a pre-light-modulation value; whether a light modulation signal mean value corresponding o the pre-light-modulation value is at a critical point of the decision voltage is determined, if yes, hysteresis judgment for the light modulation signal mean value is carried out, a final light modulation value is determined according to the hysteresis judgment result, if not, the pre-light-modulation value is taken as the final light modulation value; a brightness value of a display screen is determined according to the final light modulation value. The method is advantaged in that backlight brightness stability is realized.