To research the influence of adhesion coefficient on longitudinal train dynamics， the longitudinal dynamics calculation model for '1+1' type 20 000 t heavy haul combined train is established in this paper. The residual adhesion force is introduced to evaluate the exertion of locomotive tracting force， and the change of adhesion coefficient under influences of adhesion limit， curve adhesion dropping and curve lubrication is considered to study the difference of longitudinal train dynamic. The results show that， in the case of train tracting on straight track， the maximum tracting force of a locomotive decreases from 380 kN to 272 kN due to the limitation of adhesion， and the maximum coupler-pulling force decreases from 800 kN to 595 kN. When considering curve lubrication on R400 curve， the traction notch must be reduced to the 6th notch from 7th notch to ensure the safety of the locomotive on curve. When curve's friction coefficients are 0.075 and 0.10， the notch should be reduced to 2nd and 3rd， respectively； when curve's friction coefficients is 0.125， the notch should be reduced to 4th during leading locomotive's curve passing， the notch should be reduced to 3rd during middle locomotive's curve passing. The maximum tracting forces of locomotive are the same.So there is no significant difference in the maximum value of coupler-pulling force. It can be seen that in calculation of longitudinal train dynamics， the exertion of tracting force is closely related to the change of adhesion coefficient， insufficient adhesion between wheel and rail will limit the maximum traction force that the locomotive can exert， and then affect the coupler force.