At present, Chinese onshore fields have generally stepped into high water cut stage. The relationship between oil and water gradually becomes complex and the remaining oil is more dispersed. It’s a pressing problem to accurately reveal the distribution of the remaining oil which can provide decision-making for development adjustment measures. The core model filled with artificial quartz sand was adopted. A series of CT slice images describing microscopic pore structure and fluid distribution of the core model were obtained with the help of the micro-focus CT system. On the basis of scanned slices, three-dimensional visualization of pore structure and fluid distribution was achieved with a series of image processing methods, including pre-processing, interpolation, segmentation and three-dimensional construction. Based on the core three-dimensional pore model, this paper finds that the remaining oil in microscopic pores has different distribution features as“continuous, discrete, continuous-discrete transition”, which determines that its critical description scale from discrete feature converted to continuous feature is 2 to 7.6 times of the pore radius; on the basis of reservoir numerical simulation technology, this paper proposed the critical description scale of the reservoir macroscopic remaining oil. The research indicates that when the grid size is not higher than the critical description scale of the reservoir macroscopic remaining oil, the simulation results can effectively characterize the local enrichment phenomenon of the reservoir macroscopic remaining oil.