Viscous debris flows is the most regular and dangerous debris flows. The velocity of debris flow is the most important parameter in the dynamics parameter of debris flow. It is very important to calculate the velocity of viscous debris flow exactly and easily. The resistance of debris flow is quite different at different area: high resistance of debris flow area with low velocity; low resistance of debris flow area with high velocity. All equations of velocity of viscous debris flow at present are not good at all kinds of resistance area. The asymmetric coefficients of debris flows are quite different in different area: large asymmetric coefficients of debris flow with low resistance; small asymmetric coefficients of debris flow with high resistance. The asymmetric coefficients of debris flow could be used to classify resistance characteristics of debris flow accurately and the resistance law of viscous debris flows was got by asymmetric coefficients. By a series field observation data, an empirical equation of mean velocity of viscous debris flow was got. The velocity calculated by the asymmetric coefficients, bottom slope and hydraulic radius of flow. It is good at both high resistance and low resistance area of debris flows. It is good consistent for the measuring velocity of otherwise field observation data of debris flow and viscous mudflow by this empirical equation. The Froude number of flow is the factor of flow status: supercritical flow or subcritical flow. Ordinary viscous debris flows are supercritical flows, minorities are subcritical flows, and few are slow-motion debris flows which have too large density. The empirical equation is excellent at ordinary supercritical viscous debris flow, but it is bad for the slow-motion flow with large density of debris flow, and it is gentle large for the subcritical viscous debris flow. In the evaluation and prevention of debris flows, the mean velocity equation could be used for the velocity calculated of viscous debris flow in the channel at the upstream of the debris flow fans. At the same time, it is gentle large for the calculating the velocity of viscous debris flow on the debris flow fans. It is bad for the slow-motion debris flow, but the slow-motion could be ignored in the evaluation and prevention of debris flows.