In order to reduce the diagnostic complexity and cost of the three-level neutral-point-clamped ( NPC) rectifier, a fault diagnosis method for the open-circuit fault of power switches was proposed. This method was based on the characteristics of switching states. Firstly, the cumulative values of some switching states in half a current cycle were selected as the diagnostic variables, which were combined with the self-adaptive threshold and the phase of currents to realize the faulty bridge arm detection. Then, the outerswitch fault-tolerant control based on reactive current injection was implemented and the threshold was updated. Finally, according to the change of diagnostic variables, the faulty switch was located during fault-tolerant process. The proposed method can realize both singleswitch and double-switch open-circuit fault diagnosis without additional sensors, complicated calculations and diagnostic rules. It has the advantages of simple implementation and low cost. The experimental results verify the effectiveness and robustness of the proposed method.