An applicable and comprehensive control strategy of a natural gas/diesel dual fuel engine is presented in this paper. The dual fuel engine is converted from a conventional mechanical pump, turbo charged, heavy duty diesel engine. In the dual fuel mode, the pedal position is explained as demanded total fuel quantity, the quantity of pilot diesel and natural gas are calculated in order to provide the equal energy with the original diesel engine at the same operation condition, the proportion of the natural gas is primarily determined by the load rate and the speed of the engine. When the engine is working under light or moderate load, the intake air is throttled in order to improve the brake mean effective pressure and reduce the hydrocarbon emissions of the dual fuel engine, according to target excess air ratio and the quantities of the two fuels, the desired air mass per cycle can be obtained. After that a mean value model based feedforward control is adopted to calculate the electronic throttle position, with a universal exhaust gas oxygen sensor, a proportional-integral controller is designed, therefore feedback control is introduced to the air/fuel ratio control system to enhance its accuracy and robustness. Verification test results show that: the engine which employs the control strategy in this paper can work stably and reliably with less calibration data; the air/fuel ratio is regulated accurately and quickly; dual fuel engine has better fuel economy even though its brake thermal efficiency is lower due to the comparatively low price of natural gas; intake throttling has significant effect on improving the economy and hydrocarbon emission of the dual fuel engine under light and moderate load.