Research conclusions: As a reference，the formula of additional resistance in tunnel，stipulated in explanation of code for Design of Railway：Electric Traction (TB 1 407—82)，is available for design of tunnel section for mixed passenger and freight railway，but its simplified formula is seldom use．The gradient design for tunnel section only in accordance with Article 2 of Clause 3．2．5 in co如加r Design of Railway Line(GB 50090—2006)can not fully meet requirements of railway construction．Especially for the tunnels located in the section with severe topographical and geological conditions，the factors such as performance of locomotive，running speed of train and additional air resistance in tunnel should be considered to make full use of the mobile equipments，and the computer simulation technology should be adopted to determine the maximum profile gradient of tunnel section with formula of additional resistance in tunnel．