Microwave sintering is an advanced metallurgical technique that enables the rapid preparation of high-performance materials. Understanding the underlying mechanisms of microwave sintering through the examination of morphological evolution is crucial for optimizing the properties of sintered materials. In this study, Synchrotron Radiation Computed Tomography (SR-CT) was utilized to conduct in-situ experimental observations of the structural changes occurring during microwave sintering of metal and metal-nonmetal mixed systems. The Fourier descriptors-based method was employed to quantitatively characterize the progression of particle surface roughness. The quantitative results indicated that in the copper-graphite mixed system, the particle surface exhibited a smoother texture, and regions with a higher particle density demonstrated a more rapid improvement in surface smoothness. Through numerical simulations, it was proposed that the interaction between the material and the microwave magnetic field influenced the distribution of magnetic field intensity, thereby affecting the eddy current losses on the particle surface. Ultimately, these factors resulted in distinct evolution processes of particle surface roughness during microwave sintering.