Few studies had been done about the nature and strength of gel slug which was used to seal oil and gas in wellbore. Based on the theoretical calculation, we analyzed the stress-strain properties of gel in wellbore, and combined with gel tensile, compressive and shear experiment analysis, determined that the gel slug played the role of sealing mainly depending on the gel shear resistance which could be described by the parameter of shear strength. The calculation results showed that the strength of gel slug sealing in wellbore was directly proportional to the length and shear strength, and was inversely proportional to the wellbore diameter. The measured data had verified the theory. At the same time, a new test method of shear strength about gel was established, and the corresponding test equipment was developed. Research result provided a theoretical guidance to formula research with high gel strength and technical process design.