Ships to save fuel oil are focused in the field of marine energy efficiency and emission reduction. To reduce shipping costs, improve economic operation and increase shipping profits, shipping companies take measures to slow down main engine and burn high-viscosity fuel oil. The main engines, auxiliary engines and boilers of COSCO container vessel loading 10000 containers combust the inferior fuel oil with the high viscosity at 700cst, it is analyzed and calculated in this paper. The results show that, steam consumption to heat the inferior high viscosity fuel oil is increased. When the main engine mean speed is reduced to 50-55RPM, the ship’s actual speed is 11-12kn, navigational speed of container vessel is very low, it’s save fuel oil is very significant, fuel-efficient reaches to limit.