Abstract: Aiming at the problem of low oil pressure in a 4-cylinder diesel engine at medium and low speed, the influence of oil pump flow, oil suction resistance, piston cooling nozzle, main bearing clearance and other factors on the oil pressure in the main oil duct of lubrication system was analyzed, and the improvement scheme was put forward.After improvement, the oil pressure has been significantly improved, and the problem of low oil pressure at medium and low speeds has been solved.