The key problem of supporting structure design is the load calculation in tunnels with shallow buried depth and unsymmetrical pressure in the situation of weak surrounding rock. The results of simplified calculation method recommended by Code for Design of Road Tunnel differ from the practical situations to a certain extent because of the neglect of load calculation range that makes the results less than the practical values, which will bring about potential safety hazard. Based on the calculation method of the Code, modified computational formulas of vertical load of internal and external sides and horizontal load are put forward based on new assumption conditions and load simplified model. The modified formulas were adopted in the calculation of the entrance of Huo-Cheling Mountain Tunnel with unsymmetrical pressure, and the results were compared with those obtained by the method of the Code and numerical simulation. The comparison show that the results of modified formula coincided with those of numerical simulation, while the values calculated by the Code were smaller.