We study the electronic properties of single-layer MoS2 with biaxial tensile strain by using an ab initio method of plane wave potential technique based on the density function theory. Our results show that a small tensile strain (0.5%) will result in the transition from direct to indirect gap for ingle-layer MoS2. With the increase of strain, the feature of the indirect gap can be preserved but the gap decreases linearly. Based on the further analysis of the density of states and the projected charge density for single-layer MoS2, the reason of the change of band structure is revealed.