Cutting force is a key factor influencing the machining deformation of weak rigidity workpieces. In order to reduce the machining deformation and improve the process precision and the surface quality, it is necessary to study the factors influencing the cutting force and build the regression model of cutting forces. Firstly, the cutting parameters influencing cutting force are analyzed for LF21. Secondly, how certain cutting parameter influence the cutting component forces (Fx, Fy, Fz) are studied by the correlation analysis and the approach to choosing the right cutting parameters for machining the weak rigidity workpieces are presented. Finally, the regression model of cutting forces based on the cutting parameters is investigated in this paper.