Abstract: The system efficiency of heavy oil wells producing by mixing light oil is still low at present. The petroleum industry standard SY/T5264-2012 "Energy consumption test and calculation of oilfield production system" was studied, indicating that there were two factors which did not adapt to the evaluation of system efficiency:a) Tubing-casing annulus and fluid density difference were ignored. b) Fluid viscosity friction loss was not taken into consideration, leading to underestimated effective head lift. Two parameters, density difference and viscosity friction, were introduced to the original calculation method. The petroleum industry standard SY/T5264-2012 and the improved system efficiency evaluation method were applied in six heavy oil wells producing by mixing light oil, successfully improving system efficiency by 10.7% in average, which saved energy and reduced consumption in oil fields.