This study chose 17 real-life heavy vehicle accidents from 2010-2013 which take more than 10 deaths on average as data base to analyze sorts of basic reasons including roll stability, mass breakpoint and driving behavior when unexpected occurs.The result shows that its not just the stable roll angle value but also the test process which could play a crucial role on roll stability of heavy vehicles.The breakpoint model of mass and dimension works efficiently for over-loaded influence calculation, especially for lorries.The accident rates of the fleets where defensive driving training has applied decrease by more than thirty percent.Consequently it suggests to test the stable roll angle with the most uneven center of gravity and set the breakpoint schedule for mass and dimension of heavy vehicles.Defensive driving education and training also play a significant role in vehicle accident prevention.