Abstract: Based on lagrange equation in system dynamics,the dynamical model of the cutting arm’s swing angle run- out was derived. According to the input problem in solving the model,a calculating method of the cutting head load was proposed,and the relationships of the cutting head load relative to pressure of the driving cylinders and the swing angle of the cutting arm was obtained through simulating calculation and analysis. The simulation model was estab- lished in Simulink. In the two kinds of cutting conditions of the cutting arm’s vertical and horizontal swing,five kinds of the cutting arm’ s swing angle were selected to conduct simulation respectively,and the regulation of the cutting arm’s swing angle run-out perpendicular to the fixed cutting direction was obtained. When the cutting arm swings ver- tically,as the cutting head load increases,the horizontal swing angle run-out decreases firstly and then increases;it’ s maximum is reaching to 5. 3°,and it’s minimum is 0. 5°;as the cutting arm’s vertical angle increases,the horizontal swing angle run-out increases firstly and then decreases,which reaches to the maximum when the cutting arm’s verti- cal angle is 20°. When the cutting arm swings horizontally,as the cutting head load increases,the vertical swing angle run-out decreases firstly and then increases;it’s maximum is reaching to 5. 4°,and it’s minimum is 0. 3°;as the cut- ting arm’s horizontal angle increases,the vertical swing angle run-out decreases linearly. The regulation of the cutting arm’s swing angle run-out in the fixed cutting direction can provide important theory basis for the automatic control of the cutting arm and rectification of the cutting head’s special position.