The invention discloses a synchronous generator rotor winding turn-to-turn short circuit diagnosis method based on double coils. The method includes following steps: A, radially mounting two U-shaped detection coils on a stator iron core section inside a synchronous generator at a certain distance, parallelly connecting output ports of the U-shaped detection coils with high-resistance-value resistors with an equal resistance value, and utilizing a data collection device to collect voltage signals of the output ports of the U-shaped detection coils; B, in an operating process of the generator, processing the voltage signals of two U-shaped detection coil outlets collected by the data collection device in real time, subtracting or adding output voltage of the U-shaped detection coils, and when a result exceeds a set threshold value, judging that the synchronous generator has a rotor winding turn-to-turn short circuit fault. By the method, the defects of the prior art can be improved, and diagnosis level of the faults of this kind is elevated.