For the purpose of researching on diesel bus emission, finding out the relationship between oil consumption and emission, and establishing a diesel bus emission model, this paper first studies the diesel truck emission model using CMEM (comprehensive modal emissions model) in foreign countries, then compares the difference between diesel buses in China and standard diesel trucks in CMEM. Some parameters of the model, like speed, acceleration, quality, transmission efficiency and so on are revised, and then the calculating formula which is suitable for diesel buses emissions in China is obtained. It shows that the data calculated by the modified model fits the test data in Harbin city well.