Abstract: To address the lack of microscopic evidence regarding plastic deformation and fracture evolution in MIM 17-4PH stainless steel during tensile loading, in-situ scanning electron microscopy (SEM) tensile testing combined with electron backscatter diffraction (EBSD) was employed to investigate the evolution of surface morphology and crystallographic characteristics at different strain levels. The results showed that local plastic deformation gradually intensified with increasing tensile strain, accompanied by the progressive elongation of pre-existing residual pores. Even at a strain of 42%, no obvious macroscopic cracks were observed on the overall specimen surface. EBSD results revealed an overall increasing trend in local orientation gradients, average kernel average misorientation (KAM), and geometrically necessary dislocation (GND) density. Meanwhile, the fraction of high-angle grain boundaries increased, whereas that of low-angle grain boundaries decreased, indicating continuous changes in local lattice distortion and grain boundary misorientation distribution during plastic deformation. Fracture surface observations revealed Si-O particles within some micron-sized voids. In-situ observations further showed the formation of interfacial gaps and localized cracking between Si-O particles and the matrix. Overall, the damage and fracture of MIM 17-4PH stainless steel resulted from the combined effects of residual pore evolution and localized interfacial damage around particles during continuous plastic deformation, ultimately leading to progressive ductile fracture.