In order to reveal the fracture mechanism of shale microstructure under different circumferential pressures, uniaxial/biaxial compression tests were performed on shale microstructure based on RFPA2 D-DIP, and the shale fractures were quantified and counted using digital image technology. The results show that confining pressure has an important influence on the fracture mechanism of shale microstructure. With the increase of confining pressure, shale microstructure has two fracture modes: Ⅴ type (0 MPa, 2 MPa, 4 MPa) and inverted Ⅴ type (6 MPa, 8 MPa, 10 MPa). The microstructure fracture surface is rough under low confining pressure (0 MPa, 2 MPa, 4 MPa), and the microstructure fracture surface is flat under high confining pressure (6 MPa, 8 MPa, 10 MPa). Confining pressure has a significant influence on crack quantitative indicators. The fractal dimension decreases linearly with the increase of the confining pressure. The number of cracks and the total length of the cracks have a consistent trend with the increase of the confining pressure. They are M-shaped. The crack rate and the width of the cracks decrease with the increase of the confining pressure, and the rate of change is getting smaller and smaller.