The invention belongs to the technical field of non-destructive testing of nuclear power plants, and provides a method for detecting unequal-thickness butt welds. The method comprises the steps of: preparing a reference block, and burying multiple sets of different types of preset defects at different depths of the reference block; performing the ultrasonic parameter simulation experiment according to the features of the unequal-thickness butt weld structures; according to an ultrasonic simulation result and the features of the unequal-thickness butt weld structures, formulating parameters ofa phased array flaw detection process; for the buried multiple sets preset defects, performing process comparison experiment of radiographic inspection, manual ultrasonic flaw detection and phased array flaw detection; and according to a experimental investigation result, finally determining 100% phased array flaw detection, and performing field operation by means of a radiographic inspection casual inspection mode. The method for detecting unequal-thickness butt welds makes up the defects that the radiographic inspection is large in safety risk, many in occupation resources and easy to leak detection for the areal defects so as to improve the whole reliability of the weld joint lossless detection and ensure the unit quality safety.