In the process of manufacturing and operating of the riser pipe, the welding area between coupling and pipe will appear cracks which will reduce the strength and service life of the riser pipe. When there are two adjacent cracks in the welding area, the interaction of cracks will further exacerbate the crack propagation and cause greater damage to the structure. Here, finite element based elastic interaction between coplanar semi-circle surface cracks is examined in detail by considering a large number of identical and dissimilar crack pairs with different size and distance. Also formulas about stress intensity factor amplification coefficient of interaction cracks with respect to cracks size and distance are fitted. The research results show that with the increase of crack tip distance of two cracks, the amplification coefficient decreases by the negative exponential law, and finally approaches 1. The influence of the interaction crack size on the reference crack is close to the linear increasing trend.