Steel trusses are often used in the railway bridges.It is known that fatigue cracks may occour in the welded joints under the long-term dynamic loading. If the cracks are not detected and repaired in time, which may endanger the safety of bridges. This paper presents a solid element modeling method by the commercial finite element software package ANSYS, which can be used to model three dimensional cracked joint of bridge structures. Based on this method, the stress intensity factors are calculated. The results show that the modeling method is efficient and has a high precision for the stress intensity factor, which can be used to predict the fatigue life of the bridge.