The core of fracture splitting process was building the gap artificially. The stress concentration formed in the gap and then the initial crack generated, extended and finally ended with the fracture splitting. Taking a car crankshaft bearing block as research object, the stressstrain field of crack tip before fracturing was analyzed under different gap parameters with Abaqus software based on the criterion of cleavage fracture normal stress. The results showed that the gap was insensitive because QT500 cylinder block behaved brittle at normal temperature. Considering the problems of crack nucleation, extending, splitting groove machining and splitting load,the optimum parameters of cracking groove were determined. The experiment proved that the optimized parameters had a good effect of fracturing.