The present invention provides a decay factor accumulation method for an organic light-emitting diode (OLED) display panel with a variable refresh rate (VRR). The decay factor accumulation method includes detecting an operating frame rate of an input image; generating a decay factor compensation coefficient according to the operating frame rate and a measurement frame rate; and generating a plurality of accumulated decay factors of the input image according to a decay factor lookup table corresponding to the measurement frame rate and the decay factor compensation coefficient.