Four-stroke diesel engines burning heavy fuel oil(HFO) are often used as propulsion devices or power generators for ships. In order to improve the thermal efficiency and reduce emissions, increasing injection pressure and reducing nozzle hole diameter are the trends of injection conditions. The spray and ignition characteristics of HFO with different viscosities under high pressure and micro hole conditions were investigated. Large eddy simulation(LES) was used to solve the turbulent motion and chemical reaction kinetics were coupled. The results show that the liquid penetration length(LPL) of HFO is almost independent of the combustion, while the vapor penetration length(VPL) decreases due to combustion, and the decrease is inversely proportional to the viscosity of HFO. The spray morphology of low-viscosity HFO is similar to that of diesel, while the spray morphology of high-viscosity HFO is narrow and long, having a jagged gas-phase spray edge near the nozzle. The increase in viscosity causes the combustion phase to advance, the heat release rate to slow down and its peak to decrease, the flame lift-off length(FLOL) to shorten and the flame to appear trailing. The flame spread ability and fuel consumption rate during the steady-state combustion phase are independent of the viscosity of HFO. The Sauter mean diameter(SMD) is proportional to the viscosity of HFO, with a slight increase before ignition and a significant increase after end of injection. The decrease rate of the total surface area of droplets is higher than the decrease rate of the total volume of droplets, which is the fundamental reason for the slight increase in SMD after ignition.