[Objectives] Q690 steel is usually employed for special parts fabrication of semi-submersible lifting and dismantling platform, and its welding specification directly influences fracture mechanical properties of considered structure, as well as service life of offshore platform. [Methods] In order to evaluate fracture performance of welded joints, base material and butt welding joint of Q690 high tensile stress steel were considered as research object for uniaxial tension tests, corresponding stress-strain curves were then obtained. Based on GTN model, tensile fracture curve equations of Q690 steel and its butt welded joint were established, while corresponding calculation parameters of GTN model were obtained through exhaustive method and particle swarm method. [Results and Conclusions] Optimized values of parameters of GTN model can generally evaluate fracture performance of Q690 steel and its butt welded joint; while heuristic search algorithm based on particle swarm method can effectively improve searching efficiency with enough parameters accuracy of GTN model. Welding induced micro-defects and residual stress eventually proposed to illustrate the difference of fracture performance between Q690 steel and its butt welded joint, which also influence variation tendency of GTN model parameters.