Micropores play an important role in gas adsorption and migration. To reveal the variation rule of coal structural parameters in nanosize at different CO2 adsorption pressures, a CO2 adsorption in situ small-angle X-ray scattering test is performed in this work. The results show that the adsorption at different pressures somewhat influences the pore diameter distribution. The nanopores (2.13-33.86 nm) inside the coal samples shrink at low pressures (from 0.5 to 1.0 MPa) and swell at higher pressures (1.5 and 2.0 MPa). Regardless of shrinkage or swelling, the deformation increases with the decrease of the pore size. Ds of the samples depends upon the capillary force. The variation of D-s during 600 min (each adsorption pressure adsorbed 120 min) at five different pressures is divided into three stages. For these lower metamorphism samples (V-daf lower than 55%), the micropores are abundant and distributed unevenly, which also leads to relatively high heterogeneity on the pore surface. The variation trend of the pore size distribution with time under different adsorption pressures is correlated with the variation of D-s to some extent. This work enables us to better understand the variation of the pore size and surface fractal dimension during the process of adsorption in multi-points of pressure.