Mobility is an important parameter of tight sandstone in the process of oil and gas development, prediction, and evaluation. The complex diagenesis process and the lack of knowledge about the control mechanism of the evolution process on mobility restrict the understanding of the hydrocarbon accumulation mechanism and "sweet spot "optimization of tight sandstone. In this study, the tight sandstone of the Fuyu oil layer of the Quantou Formation in the northern Songliao Basin was taken as the research object. Utilizing a combination of high-pressure mercury injection, nuclear magnetic resonance, scanning electron microscopy, and thin section observation, the mechanism of diagenesis on the mobility of tight sandstone in the diagenetic evolution process was explored. The results show that compaction mainly occurs in the early diagenesis stage. The pore types are mainly primary pores and residual intergranular pores. Macropores and mesopores are mainly developed. Macropores are the main occurrence space of movable fluid, and the saturation of movable fluid is high. In the middle diagenesis stage A, compaction and dissolution mainly occur, and a certain degree of cementation occurs at the same time. The pore types are mainly residual intergranular pores and dissolution pores. The pore structure is gradually complicated and the connectivity is reduced. The proportion of mesopores gradually increases, and the saturation of movable fluid continues to decrease. In the B stage of the middle diagenetic stage, cementation mainly occurred. The precipitation of many cements occupied the effective reservoir space and throats such as residual intergranular pores and dissolution pores. The macropores almost disappeared, and the mesopores became the main occurrence space of the fluid, and the mobility was poor. The control of diagenesis on reservoir pore structure in different diagenesis stages determines the mobility of tight sandstone reservoirs. Finally, the mobility evolution model of the tight sandstone reservoirs under the constraint of diagenetic evolution is established. The potential relationship between diagenetic evolution, pore structure, and free fluid occurrence is clarified. All these can help a better understanding of the spatial distribution law and mobility difference of sweet spots in the tight sandstone reservoirs, and also provide a conceptual basis for further optimization of development means and technical schemes.