IEC 62305 proposed by the International Electrotechnical Commission is most commonly used. In IEC 62305, parameter P_C is defined as follows: the probability of system failure is due to buildings stroking by the lighting. The parameter depends on the protection level of Surge Protective Device (SPD). However, when lightning strokes buildings, the lightning current in steel will produce electromagnetic interference on the internal system. And when the current runs into the earth, there is a high ground potential increase. In this case, SPD will not work well. Therefore, the definition of P_C is no longer appropriate. Transient calculation models when lightning strokes metal frame of buildings have been proposed. However, no one has applied them to the study of lightning risk assessment. Here, a transient calculation model with Matlab is established to figure out transient current distribution in steels of the buildings when they are stroked by lightning and to calculate the back voltage due to ground potential rise. Under a certain approximate conditions, the effect due to lightning strokes buildings and the current goes into the earth on internal system is analyzed. Using the previous data, a clear physical model is set to show what factors affect the internal system most when lightning strokes buildings or the current goes into the earth. It is able to find that wiring mode and the installation of metal tube are the important parameters having the effect on the value of P_C. Finally, by means of determining the factor that affects the parameter of P_C and improving the parameter value, the value of lightning risk is accurately calculated with this method. The design of lightning protection and the planning of lightning protection are scientifically guided by the method.