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As a small flow, high head and low specific speed pumps, roto-jet pumps are widely used in the liquid delivery and it has very simple construction and smooth performance curve and cheap maintainment. The pressure in the chamber increase is the result of impeller and rotating shell. It has been determined according to the impeller but ignoring cylinder effect of the rotating shell for a long time which makes the calculation of pump cavity pressure inaccurate. In order to solve this problem, a mathematical model of the roto-jet pump internal pressure was established based on cylinder effect of the rotating shell and the liquid rotation coefficient was introduced. Combining experiment with numerical calculation the liquid rotation coefficient was verified and the influencing factors were subjected to sensitivity analysis. The results showed that a mathematical model can be established to calculate the internal pressure distribution of the roto-jet pump in a theoretical way. Effect of rotating shell must be taken into account of the internal pressure. The liquid rotation coefficient of the test pump was 0.75 and the theoretical pressure value at this coefficient was in good agreement with the test value. The mathematical model was verified by an example of a double-type impeller roto-jet pump. Sensitivity analysis of factors affecting showed that the wall roughness, rotation speed and flow had a weak effect on the liquid rotation coefficient. The rotation coefficient was between 0.736 and 0.764 in the test range and the fluctuation was small, which value not exceeding 3% was considered as a fixed. Tests showed that the larger the radius of the pump chamber was, the more obvious the pressure increase caused by the shell effect was, the research results can provide an important basis for internal pressure theoretical calculation of the roto-jet pump, the design of collecting pipe and the selection of installation height of collecting pipe.