Tight oil is widely distributed around the world, and industrialized exploitation has been developed and applied in North America, greatly alleviating the contradiction between supply and demand of energy. The main storage space of tight oil reservoirs is micro-nano pores, and a reliable method for studying the occurrence of tight oil in micro-nano pores has not been studied. In this paper, the microscopic occurrence of tight oils in the Songliao Basin, Ordos Basin, and Tarim Basin in China are studied in combination with nuclear magnetic resonance (NMR) and field emission scanning electron microscopy (FE-SEM) scanning techniques. A method for characterizing the storage space, occurrence state, and influencing factors of tight oil based on SEM and NMR was proposed. The results show that the storage space of tight oil reservoirs is nanoscale cracks and pores. Mainly, these are distributed on the scale of less than 1 μm and more than 10 μm, the volume of porecracks is less than 1 μm, which account for more than 80% of the total volume. This indicates that the tight oil mainly exists in the nanoscale pore throat. The scattered distribution of crude oil in the black shape of oil spot is mainly in the micro-nano pores, and the crude oil content in the micro-cracks is small, indicating that nanoscale cracks are the main space for the occurrence of tight oil. However, some micro-cracks can penetrate oil spots or oil stains, and can connect the oil spots with scattered distribution, which will greatly improve the mobility of crude oil. In addition, oil spot accumulation areas often develop illite and illite/smectite mixed-layer, and clay minerals have developed nanoscale pores. The crude oil and clay minerals have certain associated characteristics. This study is of great significance for understanding the law of the occurrence and migration of tight oil and the basic theoretical research and technological innovation of the development of tight oil reservoir.