The invention relates to a three-level inverter multi-mode fault diagnosis circuit and a diagnosis method thereof. Voltage signals of an upper bridge arm, a middle bridge arm and a lower bridge arm of each phase of bridge arms of an NPC three-level inverter are collected. A bridge arm voltage fault feature extraction module extracts fault features of the voltage signals and then sends the fault features to a neural network module to be analyzed. Voltage of each middle bridge arm is measured through a main neural network module so that eleven fault modes, including a fault-free mode, of an open circuit of a single-bridge-arm component of the three-level inverter can be diagnosed. Voltage of each upper bridge arm and voltage of each lower bridge arm are measured through two auxiliary neural network modules so that the other four fault modes of the open circuit of the single-bridge-arm component of the three-level inverter can be diagnosed, and by combination of the three neural network modules, thirteen multi-fault modes of the open circuit of each single-bridge-arm component of each phase of the bridge arms of the three-level inverter and simultaneous open circuits of multiple components of each phase of the bridge arms of the three-level inverter are diagnosed together. According to the three-level inverter multi-mode fault diagnosis circuit and the diagnosis method thereof, fault components can be exactly positioned, the algorithm structure is simplified, calculation of a Hessian matrix can be avoided, and accordingly the calculation amount and internal storage demands in a training are reduced, the operation speed is high, the diagnosis accuracy is high and the anti-interference capacity is high.