The wall-paintings of Mogao Grottoes have suffered from cracks initiated under the influence of external micro-vibration environment for a long time. Therefore, it is necessary to study the mechanism of crack initiation in the wall-paintings, which provides scientific basis and technical support for the wall-paintings protection and repair. Tensile strength is obtained by experiments of four-point bending on the plaster ground of the wall-paintings. Digital image correlation technique is used to measure the strain variation and distribution in real time. Besides, the crack initiation on the plaster ground is analyzed when tensile damage occurrs to obtain the strain limit of the wall-paintings. Moreover, the influences of initial flaw, loading method and fatigue cycle number on the bearing capacity and the strain limit of the specimen are studied. The results show that the initiation of cracks and even their development to some extent will not affect the bearing capacity of the grottoes wall structure. The initial flaw of the plaster ground can increase the risk of cracking around the flaw. Fibers such as hemp improve the plaster ground with better plasticity and durability, and reduce cracks within the fatigue limit. In order to avoid crack occurrence, maximum normal strain of the plaster ground should be strictly controlled less than 0.40%.