Abstract: A method of detecting the dryness of wet steam of a once-through steam injection boiler, comprises: first, by detecting and/or calculating, obtaining volume flow of wet steam, corresponding volume flow of saturated liquid, and corresponding volume flow of saturated vapour; and then obtaining the dryness of wet steam of the once-through steam injection boiler, X, according to the following relationship equation: X = ; wherein are the volume flow of wet steam, the corresponding volume flow of saturated liquid, and the corresponding volume flow of saturated vapour respectively. A detecting apparatus for detecting dryness of wet steam of a once-through steam injection boiler, specifically comprises: a data detecting module 1, a data processing module 2, and a data displaying module 3, wherein the data processing module 2 connects the data detecting module 1 and the data displaying module 3.