Aimed at plugging problem of baling intact rice straw by the domestic small and midsize steel-roll round baler, baling process was studied by the self-made steel-roll round baler (diameter of bale was 500 mm). The forming process of rotary straw core was essentially the process of inwinding and continuous accumulation of rice straw in baling chamber by analysis using high-speed camera. Large traction, which was produced by the rotary straw core on the subsequent feeding rice straw, was beneficial to eliminate baling plugging. Test results showed that the formation of rotary straw core mainly depended on the friction of the steel rolls on rice straw and the sustained driving force of the subsequent feeding rice straw, and the main influencing factors were analyzed. The main influencing factors were as follows: friction effect of the steel roll on rice straw, linear velocity of spring-finger and feeding quantity of rice straw. The dry mass of rotary straw core was decided as the evaluation index, and the Design-Expert software was used to optimize the level of each factor. The contributing rate of each factor on the dry mass of rotary straw core was as follows: friction effect of the steel roll on rice straw, linear velocity of spring-finger, feeding quantity of rice straw. When the parameters of sliding friction coefficient between steel and rice straw was 0.65, the linear velocity of spring-finger was 2.23 m/s and the feeding quantity of rice straw was 1.5 kg/s, the dry mass of rotary straw core was 1.58 kg. The research results can be used to support innovative design and parameters optimization of steel-roll round baler.