Out-of-plane relaxation can introduce MoS2 in flexible electronic/optoelectronic devices, while under larger strain it is possible to frustrate the structure of MoS2. On the basis of first-principle calculations, the ideal tensile stress strain relations and failure mechanism of single-layer MoS2 structure under large strain is investigated. The instability of phonon modes near the K point results in the decrease of tensile stress under large strain. The relative out-of-plane movement of Mo atoms is found to contribute to the mechanism of the soft phonon mode.