Based on Circular-Variable method, a three-dimensional coupling dynamics model of 20000-ton combined heavy haul train driving by two Harmony (HXD) locomotives was built, and train coupler force was compared in different locomotive marshalling position namely the 2+0, 1+1+0 and the 1+0+1 marshalling mode under traction and brake conditions which included the master and slave control locomotives synchronous traction, the slave control locomotive lag traction, the master and slave control locomotives synchronous brake, and brake with the controllable train tail device. The simulation results showed that under above calculation conditions, the coupler force of the 1+1+0 marshalling mode was the smallest, while under the 2+0 marshalling mode, the coupler force of the vehicles in the train and the impulse of the train were both the largest, and coupler force of the 1+0+1 mode was in middle between the 1+1+0 and 2+0 marshalling mode. In addition, when the controllable train tail device was installed on the trains, in brake condition, the coupler force of the train were all smaller compared to which of no train tail device. Thus, while marshalling for the 20000-ton combined heavy haul train, the 1+1+0 mode with controllable train tail device would be the best marshalling mode.