The invention discloses a method for fault diagnosis of a three-phase full-bridge rectifier circuit thyristor. The method comprises steps of: 1, constructing a thyristor fault sample set: 101, determining the quantity D of thyristor fault state types in a three-phase full-bridge rectifier circuit; 102, acquiring each type of thyristor fault state data and expressing a matrix space; 103, constructing a matrix space sample; 2, classifying and positioning a thyristor fault: 201, extracting a fault vector beta; 202, solving the distance d(beta, w) from the fault vector beta to the matrix space sample Si; 203, completing the calculation process of the value of d(beta, w); and 3, synchronously outputting a thyristor fault processing result. The method is novel in design; the fault is classified and positioned by adopting a method of vector-to-subspace distance, so that the method is high in precision, small in calculation amount, intuitive, quick and effective, overcomes the defect of weak normalization capability of a neural network detection method in the prior art, and eliminates local minimum points.