Abstract: This paper analysed the kinematic properties of ball joint hinged on the piston rod and steering drag link of a separated assistant steering mechanism, and the equations of the path and mathematical model of pressure angle were established. The changing regularity of pressure angle of ball joint and the optimum position region of pressure angle in given rod structure were proved through computer aided analysis. It is indicated through the analysis of assistant steering mechanism of the farm transporter model FR1608 that within the range of standard swinging of the pitman arm, the design rod structure satisfies the requirement of optimum pressure angle.