Abstract: A large number of unevenly distributed bending cracks exist in engineering structures, which have great influence on the stability of structures. To obtain the stress intensity factor (K) values of the bending crack tips easily and quickly, the bending cracks are approximated as equivalent straight. By calculating the equivalent straight crack tip K value, the bending crack tip K value is obtained. Based on the existing approximate methods (horizontal projection method), three new approximate methods are proposed, which are vertical projection method, center rotation method and line method. The comparison of the three new approximation methods with the existing ones shows that the approximation process of obtaining equivalent straight cracks is optimized. The application range of approximate calculation method is modified and expanded. When calculating the main crack tip K of the bending crack, when the main crack is perpendicular to the load direction and the angle between the branch crack and the load direction is less than 45°, the vertical projection method is the best approximation method; but when the angle between the branch crack and the load direction is greater than 45°, the central rotation method is the best one. When calculating the branch crack tip K_Ⅰ of the bending crack, the horizontal projection method is the best approximate method. When the branch crack tip K_Ⅱ is calculated, the vertical projection method and the center rotation method are superior to the horizontal projection method in the range of the length ratio (b/a) of the main and branch cracks of the bending crack less than 0.3. Compared with the horizontal projection method, the line method is more suitable for the calculation of K_Ⅱ value when the angle between the main crack and the direction of load is small.