The physical mechanics and velocity of debris flow are crucial for debris flow mitigation measures. The two aspects closely relate to the grain composition, density, and flow depth. We present a combined research on Manning coefficient, debris flow classification, and mean velocity using mechanical analysis. Comparison of Manning coefficient reveals that it varies greatly at the same observation site and given event. The reciprocal Manning coefficients for viscous flows in Jiangjia Ravine, China, are the highest among the observation sites at a given flow depth. The stony debris flows in Kamikamihorizawa, Japan, are mainly governed by inertial stress, whereas the viscous debris flows in Jiangjia Ravine and Wudu, China, are mainly governed by friction stress. The reciprocal Manning coefficient of stony debris flows in Kamikamihorizawa, but not of viscous debris flows in China, increases with increasing Savage and Bagnold numbers. The reciprocal Manning coefficient decreases with the Friction number for both viscous and stony debris flows. Based on dimensionless parameters, we proposed debris flow classification from the perspective of physical mechanics including friction- and inertial stress-dominated flows. Finally, a new debris flow mean velocity equation was developed considering the characterizing diameter parameters (D₁₀, D₅₀), density, flow depth, and channel gradient. This equation performs well and could be updated in the future if the observed data of friction- and inertial stress-dominated flows are available. The results of this work can help strengthen the resistance of debris flows in different flow regimes.