The channels of natural viscous debris flows are all compound. The hydraulic characteristics (wetted perimeter, overflowing area and hydraulic radius) are different under different width ratio of beach to groove and different slurry depth of debris flow. By using the hydraulic calculating method of natural river, this paper discusses the hydraulic characteristics in compound channels of viscous debris flow. The results indicate that the width ratio of beach to groove has a great influence on overflowing capacity and discharge distribution of beach and groove. When the width ratio is the same, the groove discharge decreases with the increase of slurry depth, while the beach discharge increases; when debris flow with the same scale passes by channels which have different width ratio, the overflowing capacity in channels with bigger width ratio is much lager than the smaller one’s; the influence between beach and groove in debris flow compound channels can be expressed by COH and it decreases with the increase of debris flow discharge; when debris flow with the same scale passes by compound channels, the bigger the width ratio is the greater the influence between beach and groove is.