During the crack propagation process, the crack branching behavior makes fracture more unpredictable. In this work, three kinds of crack branching models were selected, and the FEM simulations based on GTN（Gurson-Tvergaard-Needleman） damage model were used to further investigate the influence of constraint on the crack branching behavior during crack propagation. The results show that the constraint has a great influence on crack branching behavior. Compared with low constraint, the crack branching phenomenon is more obvious under high constraint. The matching between strength mismatch induced by both the mechanical properties and dimensions of different regions and constraint is the key of crack branching. When the crack propagation trends induced by strength mismatch and constraint different in direction, similar in size and large enough, crack branching occurs.