Abstract: Based on the detailed mechanics analysis of a rotary drilling rig under drilling-bucket-lifting conditions, a mathematics model was established. The factors influencing the loads of the mast hydraulic cylinder and the lift arm hydraulic cylinder were analyzed respectively. A simulation model based on MATLAB was built up and was simulated in Simulink. The results indicate that, under drilling-bucket-lifting conditions, the load of the lift arm hydraulic cylinder has nothing to do with the acting line of all other external loads except the weights of the connecting rod and the lift arm; the change of lift arm angle has the most impact on both of the loads of lift arm hydraulic cylinder and the support reaction force of the hinge connecting lift arm and swing platform. The simulation data are basically consistent with the experimental ones, which show that the mathematics and simulation model can well describe the mechanics behaviors of the rotary drilling rig.