The invention discloses a generator-rotor inter-turn short circuit diagnosis device based on excitation step method. The device comprises a direct current constant voltage source and data acquisition analyzers. The direct current constant voltage source is connected to an excitation winding through a first program control switch. A sampling resistor is connected in series to the excitation winding. The data acquisition analyzers are connected in parallel to two ends of the sampling resistor. The data acquisition analyzers determine whether a motor rotor winding generates an inter-turn short circuit through collecting an excitation current of the rotor winding. Two ends of the excitation winding are connected in parallel to a de-excitation resistor through a second program control switch. By using the device and the method of the invention, disadvantages in the prior art can be overcome, and an off-line diagnosis level of a turbonator rotor winding inter-turn short circuit fault is improved.