Abstract: Considering the low thermal efficiency, high excess air coefficient and low load rate of the in-service water jacketheater of Shengli Oilfield, the relationship of the load rate, excess air coefficient and fouling resistance with the thermalefficiency of the water jacket heater was calculated with the calibration and air-conditioning calculation software for waterjacket heater, and on this basis, the fouling resistance of the in-service water jacket heater was calculated reversely. Accordingto the calculation, the dissipation heat loss increases and the exhaust heat loss decreases with a decrease of load, but thetotal heat loss varies a little. So, the load rate has little effect on the thermal efficiency of water jacket heater. The exhausttemperature increases with a rise of excess air coefficient, and under the combined action of two factors, a considerabledecrease in thermal efficiency of water jacket heater will be caused. The fouling resistance of the in-service water jacketheater increases obviously, which makes the exhaust temperature and water bath temperature significantly higher than thedesign values, further resulting in the decrease in thermal efficiency of water jacket heater. Thus, the key measures to improvethe efficiency of water jacket heater are developed, including timely cleaning the fouling, replacing the boiler water regularly, installing flue gas analyzer, and replacing automatic regulating burner.