The dynamic simulation technology for a powertrain test bench is discussed in this paper. Dynamic emulation of mechanical loads is the key technology of laboratory tests in cases when the test site is moved from the road to an indoor laboratory test bench. The combustion engine is replaced by an electric drive motor, which replicates the torque and speed signature of an actual engine. The road load resistance of the vehicle on a real test road is accurately simulated using a load dynamometer motor. In this study, we analyzed and compared the advantages and disadvantages of the inverse dynamics model and the forward model based on the speed closed loop control method, from the perspective of the high-order, nonlinear, and multi-variable characteristics of the test bench system. We developed a load simulation method based on the speed adaptive predictive control (SAPC). It avoids the complex algorithm of closed loop speed compensation and reduces the effects of inaccurate model parameters on the control precision of the simulation system. On the test bench, the vehicle start and dynamic shift process are simulated, and the test results are presented here to demonstrate successful emulation.