A method of star-tube combined segmented grain is proposed to improve the combustion performance of hybrid rocket motor. The star-tube combined segmented grain consists of a single-port star part and a single-port tube part. A mid-chamber forms between the fore-grain and the aft-grain for better mixing effect. The single-port feature gives hybrid rocket motor several advantages, such as simple structure, high reliability, and variable combinations. This paper is mainly aimed at studying the combustion characteristics of hybrid rocket motor with star-tube segmented grain through three-dimensional steady simulations. Combustion performance of the motors with different segmented grain combinations, including fore-tube/aft-tube, fore-tube/aft-star, fore-star/aft-star and fore-star/aft-tube, is contrastively analyzed. The motor in this paper adopts polyethylene and 90% hydrogen peroxide as the propellants. Simulations reveal that segmented grain with different-type grain combinations could greatly change the flow field in the second half of the combustion chamber. Transformation of the flow field is beneficial to the mixing between the fuel and the oxidizer, and it could increase the fuel regression rate and the combustion efficiency. The turbulence effect of tube aft-grain is better than that of star aft-grain. Among the four segmented grain combinations, the combination of star fore-grain and tube aft-grain is the preferred method with optimal overall performance. This grain configuration could increase the regression rate of tube aft-grain to surpass that of star aft-grain in other combinations. Besides, hybrid rocket motor with this grain configuration achieves the highest combustion efficiency.