The invention relates to the manufacturing technology of a single cylinder high speed diesel engine with a small cylinder diameter, in particular to a constant pressure oil outlet valve suitable for a diesel engine fuel oil system. The constant pressure oil outlet valve comprises an oil outlet valve tightening seat, an oil outlet valve spring, an oil outlet valve core, an oil outlet valve seat, a steel ball, a spring seat and a pressure relief valve spring, wherein the oil outlet valve spring, the oil outlet valve core and the oil outlet valve seat are mounted in the oil outlet valve tightening seat, the steel ball, the spring seat and the pressure relief valve spring are sequentially assembled in the oil outlet valve tightening seat, the oil outlet valve tightening seat is provided with a lift-range limiting cavity capable of controlling the lift-range of the oil outlet valve, the oil outlet valve core and the oil outlet valve seat are tangentially matched to form a one-way oil supply valve, the oil outlet valve core and the steel ball are tangentially matched to form a constant pressure relief valve, and the lift-range limiting cavity can influence the throttle flow and the initial pressure of the fuel oil in the oil outlet valve through controlling the jumping height of the oil outlet valve core. The constant pressure oil outlet valve has the functions of limiting the lift-range and relieving pressure, can eliminate the secondary injection, the unstable injection generated easily due to low speed and the cavitation erosion easily occurring to a high pressure chamber, and the like, and has the characteristics of simple structure, skillful process, favorable performance, low fuel oil consumption rate, low discharge, stable injection, an the like.