The remaining oil exists as discontinuous phase in the ultra-high water cut period, which is greatly influenced by microscopic force. Taking the residual oil in the droplet as the object of study in the ultra-high water cut period, the micro mechanical cause of oil droplet in the capillary model is analyzed, and it is clarified that the capillary force is the main force formed by the residual oil. The model of two hole pore structure is selected to establish the critical condition model of the oil droplet transfer tunnel radius, and the mechanism of choosing the remaining oil droplet capillary channel is studied. Quantitative analysis results show that the driving force and the viscous force have a great impact on the minimum boundary value of the channel radius passing the oil droplet. The results can provide theoretical guidance for analyzing the conditions of remaining oil in ultra-high water cut period and enhancing oil recovery.