In using polymer products, people are most concerned about their failure conditions, and the important manifestation of failure is the yield of the material. So far, the dislocation theory has been widely used to explain the yielding phenomenon of polymer materials. This theory usually focuses on the crystal orientation and destruction while ignoring the crystal deformation and the stress acting on the crystal. The orientation and destruction of the crystal are only the results of the yielding, and the ability of the crystal to withstand stress is the direct cause of yielding. Therefore, this paper will start from the stress and inhomogeneous deformation of crystals to study the yielding behavior of polymer products, hoping to provide new ideas for understanding the failure behavior of polymer materials. Here, the widely used isotactic polypropylene (iPP) material is selected as the research object, and iPP samples with different lamellar thicknesses are prepared by isothermal crystallization of iPP melt at different temperatures. Two-dimensional wide-angle X-ray diffraction spectroscopy was used to monitor in situ the crystal destruction and crystal orientation processes of the iPP samples during stretching. The “covering method” was used to process the two-dimensional X-ray diffraction patterns for the first time, the change of the 2θ angle of the (110) crystal planes during the stretching process was observed in situ, and the deformation of the crystal in two directions (parallel to the stretching direction and perpendicular to the stretching direction) was distinguished. The results show that for iPP crystals with different lamellar thicknesses, the inhomogeneous deformation of the crystals during the uniaxial stretching process is a common phenomenon; the destruction and orientation of the crystals always occur at the same time, starting from the yield point, which is independent of the lamella thickness; the critical stress corresponding to the crystal destruction is related to the thickness of the crystal. The thicker the lamellae and the more stable the crystal, the greater the critical stress required. The above results show that in situ X-ray diffraction spectroscopy can observe the crystal structure changes during the stretching process in real time, thereby directly correlating the crystal structure evolution with the macroscopic mechanical properties.