Disclosed is a method for compensating a pixel driving circuit of an OLED display panel. In the compensation method, a driving transistor is enabled to operate stably in a saturation region for twice, and a threshold voltage of the driving transistor is calculated based on a collected charging voltage and charging time. A pixel driving circuit is compensated by establishing a threshold-voltage compensation table. The compensation method is easy to operate and can significantly improve a detecting speed of a threshold voltage. Moreover, an effect of a voltage-current conversion factor on detecting accuracy of a threshold voltage can be avoided.