An ultrasonic method for measuring residual stress of a cambered workpiece comprises the following steps: S1, performing stress testing to obtain a function curve t0x=M (x) between a propagation distance x and the propagation time t0x of a longitudinal critical refraction wave in a zero-stress plate calibration block; S2, performing stress testing to obtain a function curve Kx=F (x) of a nominal acoustic elasticity coefficient Kx corresponding to the propagation distance x; S3, measuring the propagation time t of the longitudinal critical refraction wave in a cambered workpiece; S4, measuring a propagation distance a of the longitudinal critical refraction wave in the cambered workpiece; S5, according to the function curve t0x=M (x) and the function curve Kx=F (x), obtaining the propagation time t0a and a nominal acoustic elasticity coefficient Ka corresponding to the propagation distance a in a zero-stress state, and then according to a formula sigma=K (t-t0), working out the residual stress of the cambered workpiece to be measured. By the ultrasonic method, the residual stress of the cambered workpiece can be measured conveniently and fast; meanwhile, the problem that in the prior art, calibration is inaccurate is solved, so that the ultrasonic measurement accuracy of the residual stress of the cambered workpiece is greatly improved.