Experimental studies were carried out on real rear-axle housing in a farm transport vehicle to investigate the influence of low amplitude preloading on structure life，in order to explore increase potential of rear-axle housing in a farm transport vehicle fatigue life. It is proved that a strengthening area exists in low amplitude loading area. Fatigue lives of rear-axle housings have been increased in some extent with preloading within this area. It can make fatigue life double at most. Based on physical tests, the best strengthening load region of rear-axle housing in a farm transport vehicle was estimated. It provides some technical supports for increasing fatigue life of real rear-axle housing.