The invention relates to an automobile constant speed engine specified load detection method, which defines the rotating speed range of the automobile speed under the engine rated torque and the full-load minimum fuel consumption rate when the high speed gear is engaged in a gearbox on a chassis dynamometer by an automobile, after the test and the calculation, the load constant force value of a power absorbing device of the engine defined load is determined at the defined automobile speed point, when the load working condition is defined by the engine delta rated torque, the load is controlled by the constant force of the power absorbing device, the accelerator is adjusted and stabilized, and the vehicular dynamic property and fuel economy are detected in the constant speed at the defined automobile speed point, the method has good accuracy, validity and operability, meanwhile, the method can avoid the damages to the in-use vehicles or engines with worse technology condition, thereby increasing the detection safety.