The forward slip coefficient is one of the most important parameters for heavy rail rolling using universal mill.For simplifying the theoretical model,the vertical roll with box pass was simplified as an equivalent flat roll first.Second,the neutral angle of horizontal roll and vertical roll was represented.Then,the equation of neutral line on the flank of horizontal roll was determined and the forward slip coefficient of the web was derived according to different positions of neutral line.Finally,the forward slip coefficient of the top and base of heavy rail was obtained.The theoretical results were basically in agreement with the experimental data. The forward slip coefficient is one of the most important parameters for heavy rail rolling using universal mill. For simplifying the theoretical model, the vertical roll with box pass was simplified as an equivalent flat roll first. Second, the neutral angle of horizontal roll and vertical roll was represented. Chen, the equation of neutral line on the flank of horizontal roll was determined and the forward slip coefficient of the web was derived according to different positions of neutral line .Finally, the forward slip coefficient of the top and base of heavy rail was obtained. the theoretical results were basically in agreement with the experimental data.