In order to explore the cutting laws of difficult-to-machine material 27SiMn steel, an orthogonal method is employed in its cutting experiment by using carbide tool. The relationship between cutting process parameters (when cutting depth is 0.3～0.8 mm, feed rate is 0.08～0.2 mm/r, and rotating speed is 200～400 r/min) and three dimensional cutting forces is revealed, and the empirical formulas of three dimensional cutting forces are deduced by using the linear regression method. Experimental results show that cutting depth is the most influential factor in cutting force, followed by feed rate and cutting speed. Cutting force is positively correlated with cutting depth and feed rate, but when rotating speed is over 320 r/min, the cutting force decreases. The cutting force effect of empirical formula is verified through significance test.