One of the key issues facing the low compression ratio engine is the start difficulty, especially at high altitude. And coolant temperature is an important parameter, which would affect misfire and instability of diesel engine combustion during start. An experimental study of heating coolant temperature to improve start performance and effect of coolant temperature on the combustion characteristics of low compression diesel engine during start at different altitudes was carried out. Tests were led on a mid-speed heavy-duty intercooled-turbocharged diesel 14.25:1 CR (compression ratio) engine, whose intake and exhaust pressure was controlled by the plateau simulation test system to stimulate the altitude conditions including 0 m, 3000 m and 4500 m. The results indicated that, the effect of coolant temperature on the start performance and combustion characteristics of low compression diesel engine was more significant at higher altitude. There was a -3.72%, 14.83% and 48.74% average reduction in the time of start-up period for every 20 ? rose in coolant temperature at the altitude of 0m, 3000m, 4500m. The main pattern of combustion during start of different coolant temperatures was single parent peak under plain condition (0 m) with stronger combustion stability and better acting ability. But at the altitude of 3000 m and 4500m, when the cooling water temperature was low, there was an ignition difficulty in the cranking period, and there are misfire cycles and larger pressure fluctuation in the start-up period. The ratio of the single parent peak cycle was increased and the cycle of misfire and afterburn disappeared in speed-up phase with the increase of coolant temperature, the IMEP (Indicated Mean Effective Pressure) was increased and the speed-up phase was shortened. There was a 2.78%?18.52% and 29.76% average increase in the ratio of the single peak type cycle of start and a -0.05bar,1.52 bar and 1.91 bar average increase in IMEP for every 20 ? rose in coolant temperature at the altitude of 0 m, 3000 m and 4500 m. Regarding the combustion characteristics of idle condition, there was a 2.14°CA, 3.85°CA and 4.5°CA average advance in The crank angle position for peak pressure and a 4.79%, 6.06% and 7.38% shortening in combustion duration for every 20 ? rose in coolant temperature at the altitude of 0m, 3000m and 4500m.