When exceptional length goods are transported by railway, the protruding part of the goods will lead to an increase in the vertical dynamic load, which is not conducive to the safety. Considering the future application needs, the allowable load of 80t railway flat wagons for loading exceptional length goods is studied. Based on SIMPACK to establish the NX80 flat wagon model and the reliability of the model is verified through actual experiments conducted by the original Ministry of Railways. After determining the worst running conditions of the vehicle, the vertical load generated by exceptional length goods in different weights and lengths is obtained. Based on this, the allowable load of NX80 flat wagons for transporting exceptional length goods of different lengths is proposed, providing technical support for the safe use of 80t railway freight cars in the future.