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According to the structural characteristics of new-type horizontal diesel engine, the cooling jacket of forced cooling closed-loop system was designed, the temperature and pressure of inlet position and key points of cooling jacket were measured and analyzed under different working conditions. Computational fluid dynamics (CFD) software was utilized to study the fluid flow and the flow field, the pressure field and heat transfer coefficient distribution were analyzed and optimized. The results indicated that the mean flow velocity, average heat transfer coefficient and whole pressure loss of the original cooling jacket were 1.00m/s, 7767W/(m2·K) and 0.027MPa respectively. They met the requirements of engineering design. But the cooling water flow velocity and average heat transfer coefficient between two cylinder bore were non-uniform. There were big swirls in the central region in public cavity and the top region in the second cylinder block water jacket and dead flow region in the bridge of the nose area of the second cylinder head and the local area below two exhaust port. After optimization,the mean flow velocity and average heat transfer coefficient came up to 1.35m/s and 9826W/(m2·K), increased by 35% and 26.5% respectively than the original design. The mean flow velocity of cooling water came up to 1.33m/s,increased by 41.5% and the average heat transfer coefficient was greater than 5000W/(m2·K) in the bridge of the nose area of cylinder head. Compared to the original design, there was no dead flow region, no big swirls existed in new cooling jacket.