The output power of diesel engine is one of its important performances. For the individual components of an engine, the accurate output power can be measured through the dynamometer. However, it is not easy to accurately measure the output power of the diesel engine during tractor ploughing. Based on the torque model of the diesel fuel control strategy, the output power of the diesel engine was measured online during the Lovol tractor ploughing process with the theoretical speed of 5.7km/h and 8.9km/h. The indicated power of the torque model was compared with that of the combustion analyzer. The test results showed that when the tractors ploughing speed was relatively stable, the error of the average value of the indicated power was 2.99%, but the instantaneous value became distorted seriously. It was found that the changes in the fuel injection amount and the injection timing are the main reasons for the large deviation in power measurement. To improve the accuracy of the online measurement for diesel engine output power, the quantitative relationships between the fuel injection quantity, fuel injection time and torque under 12 typical working conditions were obtained by using the GT-Power model. Then, the correction coefficients of fuel injection quantity and fuel injection time were used to optimize the original measurement method. The optimized results showed that under the stable operation, the average measurement error of diesel engine output power is 0.88%, and the instantaneous value’s error was reduced significantly. It can be learned that optimizing the fuel injection strategy of the diesel engine is an important way to improve the performance of the diesel engine during tractor ploughing. 