Abstract: In order to explore the actual force and motion states of the hammer while hammer mill running, and further to provide analysis basis for the dynamic characteristics of hammer mill rotor, force and motion states of hammer at the unloaded running were studied by methods of dynamic analysis and critical analysis in precondition of the friction between the hammer and its pivot was taken into consideration. The theoretical analysis results indicated that there existed a random static deflection phenomenon for the hammer relative to the rotor radial line passing the central point of hammer pivot, and this phenomenon could be realized just in the range of the critical deflection angle which was determined by the static friction coefficient between hammer and its pivot as well as the key structure and working parameters of hammer-rotor system. Furthermore, the random static deflection of all hammers on the hammer mill rotor could make a random change for the centroid position of the whole hammer-rotor system. The results provide a theoretical basis for the dynamic characteristics analysis and structure optimum design of the hammer mill.