In order to solve the cold start difficulty for diesel engines on the plateau，an experimental study on the effect of flame intake preheating on the starting performance of diesel engine at 5 km altitude was carried out on a plateau environmental test chamber. The results show that under the altitude of 5 km and the intake temperature of 303 K without intake preheating，the diesel engine falters in the speed range of 400—500 r/min at the coolant temperature of 303，318 and 333 K，respectively. At the coolant temperature below 318 K，the diesel engine falters for a longer time and even cannot go to idle condition. After analyzing，it is found that at 200 r/min，large amount of residual gases from the last cycle results in misfire. At 400 r/min，no misfire happens. When the engine speed goes above 500 r/min，the misfire cycle rate increases significantly due to the increased ignition delay period， leading to falter at 400—500 r/min. Under the altitude of 5 km，when a flame intake preheating device was used and set its preheating power to 0.84 kW at 200 r/min and 400 r/min，1.25 kW at 600 r/min and 2.1 kW at 800 r/min， respectively，the misfire cycle rate is dropped to 0% across all speeds. Experimental results show that the flameassisted intake preheating system used with a precise fuel control can effectively improve the starting performance of the diesel engine at plateau，reducing the engine starting time to within 4 seconds at the altitude of 5 km.