The coupler is an important component of the heavy-haul train, and the coupler force has a significant impact on the running safety of the heavy-haul train. In order to analyse the coupler force of 20,000-ton heavy-haul train under different working conditions, based on the train longitudinal dynamics theory, train control method and the dynamic modelling method of the hook retarder, the longitudinal dynamics model of 20,000-ton heavy-haul train assembled by HXD1 locomotive and C80 coal haulage converters was established to analyse the coupler force distribution law of heavy-haul trains running though different traction force, slave control locomotive lag and different coupler clearance. The results show that: under traction conditions, The lifting speed of traction force has no obvious effect on the steady-state coupler force, while the increase of traction gear significantly increases the steady-state maximum coupler force. The traction gear should be reduced while ensuring the normal traction and starting of the train. With the increase of starting time difference between the master locomotive and the slave locomotive, the pull force in the middle and tail of the marshelling vehicle increases, so the simultaneous operation of the train should be guaranteed as much as possible. The clearance of coupler has obvious influence on the tension of coupler during the start of heavy duty train. Attention should be paid to the maintenance of coupler clearance, and the coupler force can be reduced with the use of traction bar.