The invention relates to rapid online diagnosis method of high-power rectifier open-circuit fault, including: 1) collecting actual output direct current value id(t0) and three phase input electric current instantaneous values ia(t0), ib(t0) and ic(t0) of rectifier at t0 moment, and calculating theoretical output direct current value id′ of the rectifier according to ia(t0), ib(t0) and ic(t0); 2) comparing id′ (t0) and id(t0), when id′ (t0) is not more than k1×id(t0), comparing absolute value of maximal three phase input electric current instantaneous value and pre-set threshold value A1, if the maximal three phase input electric current instantaneous value is not less than A1, the rectifier appears open-circuit fault. The inventive method is simple, with no complicated step for performing fault diagnosis, of course, the short fault diagnosis period and tripping protection occupation time is short, but it also can protect the rectifier effectively and prevent equipment damage from longtime working under fault condition.