Abstract: Starting with the analysis of the working characteristics of a front mounted beam-pumping unit, we discussed in this paper the optimization of main-body parameters of this unit according to the present theory and method of optimum design, we have proposed the design objective by reducing the maximum fluctuating value of net torgue of output shaft in the reducer, so as to determine main-body mechanism size, balance weight and offset angle of balance mass and to make rajah-body mechanism design and calculate carrying capacity for various series of pumping units.