Abstract: In the ocean waveguide, the sound field with light and dark interference fringes can be observed in the spatial- frequency domain. Chuprov defined waveguide invariant according to the interference fringes in the r-ω domain. For a long time, researchers have not considered the effect of source-receiver azimuth angle on the value of waveguide invariant, and the effect of azimuth angle on interference fringes has been ignored in traditional sound field interference studies. In this paper, it is found through experiments and simulations that the azimuth variation has a significant effect on interference fringes for the common horizontal slowly varying wedge waveguide in the offshore continental shelf of China. Therefore, the definition of waveguide invariant given by Chuprov, is extended in the case where the influence of azimuth variation on interference fringes is considered, and it is defined as the generalized waveguide invariant. Through theoretical derivation, numerical simulation and sea trial data analysis, it is found that the generalized waveguide invariant can well explain the acoustic field interference phenomena in the numerical simulation of the horizontal depth slow-varying wedge waveguide and in the offshore experiment, while the traditional waveguide invariant fails to do so.