The relationship between the forming process of serrated chip and the cutting force in the cutting process of titanium alloy TC4 was studied, and the micro plastic deformation zone in the formation of serrated chip was analyzed.The experimental results indicate that the chip is changed from strip chip to serrated chip while the cutting speed is more than 48.75 m/min. The serrated chip formation process leads to the fluctuation of cutting force in the cutting process. The variation of cutting force is consistent with the variation of serrated chip. The width of the plastic deformation zone decreases with the increase of the cutting speed.