To fully understand the solidification effect of metakaolin (MK) on Pisha sandstone cement soil, unconfined compressive strength (UCS), ultradepth 3D microscopy (U3DM), nuclear magnetic resonance (NMR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) tests were carried out on the solidified soil. The solidification mechanism of MK was comprehensively analysed from the perspectives of macromechanics, mesostructure and micromechanisms. From the perspective of macromechanics, the damage variable V tended to decrease with increase in MK content, which indicates that the addition of MK helps improve the ability of the solidified soil to resist damage. From a mesoscopic perspective, the addition of MK reduced the average height difference of the surface of the solidified soil. The complexity of the size, number, distribution and shape of pores in the solidified soil was comprehensively analysed based on NMR experiments. With increase in MK content, the proportion of pores with a pore radius >1 µm decreased, whereas the fractal dimensions DT and D2 increased. XRD and SEM experiments show that the hydration products (calcium aluminate hydrate and calcium silicate hydrate) formed after the incorporation of MK were tightly combined with the soil particles and filled the pores. A grey entropy correlation analysis carried out to analyse the correlation effect of different pore structures on the strength of the solidified soil showed that pore sizes of 0–0.1 µm had a large influence on the UCS. This is a valuable finding on the link between the microstructure and macromechanics of solidified soils.