The invention discloses a quenched steel decarburized layer depth measuring method. A quenched steel sample is made into a cross section metallographic sample through cutting, inlaying, grinding and polishing, a wavelength dispersion spectrometer is utilized to perform measurement of a carbon element characteristic X ray K[alpha] ray on the cross section metallographic sample, voltage of 10 to 20 kV, beam current of 100 to 1000nA, measuring step length of 10 to 20 [mu]m and measuring time of 0.1 to 5s are adopted, and a measuring point extends from a cross section metallographic sample surface layer towards a matrix; and sample edge distance corresponding to a transformation node position between a background signal and gradually increasing signal intensity is the thickness of a fully decarburized layer, edge distance corresponding a transformation node position between gradually increasing signal intensity and gradually stable signal intensity is the depth of a total decarburized layer, and a difference value of the total decarburized layer and the fully decarburized layer is the depth of a partially decarburized layer. The quenched steel decarburized layer depth measuring method is suitable for measurement of the depth of a quenched steel decarburized layer which cannot adopt a metallographic method, and is higher in accuracy than carbon content measuring methods such as a hardness method, a chemical method and a spectral analysis method.