As one of dominate geological disaster and geomorphologic processes in mountains, debris flow dynamics is essential in designing of structures against debris flow disaster, risk appraisal and mitigation of the disaster. Essential principles of debris flow dynamics are composed of flow velocity, impact and abrasion of debris flow. Based on two-phase theory, both continuum and momentum equations every phase are established. Further, method to calculate velocity of two-phase debris flow is established by introducing parameters M and G0 . Impact forces and duration can be identified by impact formality on banks of debris flow valley or structures. According to abrasion forces of slurry and particles in debris flow, methods to calculate abrasion thickness and velocity are provided primitively. These principles have applied effectively in engineering since 2000.