Abstract: In this paper,started from motor,the screw transformations their inverses anddifferetial relations are discussed in detail,Based on Newton-Euler equations,a set ofmotor equations useful in robot dynamics analysis and calculatiion is presented.The derivation is sim-ple,and the presented robot dynamic model is concise and can be easily programmedin computation.The theory in the paper reveals a new way in dynamic and kinematicstudy of other spatial mechanisms.Lastly the correctness of the theory and mathematic derivation has been confirmed by an example.