Abstract: Ti40 burn resistant titanium alloy is one kind of beta phase titanium alloy. There existed complex cutting deformation, higher cutting temperature, higher cutting force and fluctuations, and serious tool wear and damage in the process of machining Ti40 alloy. In this paper, the chip formation characteristics, chip formation mechanisms and their relationships with dynamic fluctuation force of Ti40 beta titanium alloy are analyzed. It is shown that shear bands are formed inside the deformation of each beta phase grain, its chip thickness varies irregularly, and there exists micro-serrated teeth and micro-cracks on the serrated chips. With cutting speed increasing, the formation frequency of adiabatic shear bands is lifted, notable serrated teeth are formed and the sub-grains are formed more obvious and fine among adiabatic shear bands. At peaks of milling force, notable fluctuation is shown, and the fluctuation times and amplitude are corresponded to the formation of notable serrated teeth. The fluctuation of milling force is caused by adiabatic shear and cutting direction change due to vibration.