The core-filled composite pipe pile is composed of the prestressed high-intensity concrete pipe pile (PHC pile) and ordinary reinforced concrete inside it, which can be helpful to enhance the bending resistance of the pipe pile. For the new composite pile, the calculation formula of the total bending capacity is derived according to static equilibrium conditions of the pile cross-section. Based on the strain difference at the interface between the inner circular and outer ring areas of the pile, the equivalent rectangular stress diagram coefficients in the current specification suitable for rectangular cross-section are improved. So, the computation method of the stress diagram coefficients suitable for the composite pile with circular cross-section are established. The research results show that the results obtained using the proposed theoretical formula agree well with the tests. The pile bending capacity is obviously influenced by the inner reinforcement ratio and tension stress on the prestressed reinforcements. The bending capacity increases nonlinearly with the reinforcement ratio and decreases linearly with the increase of the tension stress. The sensitivity of the bending capacity to main factors follows the order: outer diameter > outer reinforcement ratio >ring thickness > inner reinforcement ratio > tension stress. The core filling can improve the pile bending capacity by 26% and 68% under the inner reinforcement ratio 1% and 3%, respectively. The reason for reinforcing effect of core filling is that the filling can strengthen the limit state and reduce the height of neutral axis on the cross-section, which leads to the significant increase of the bending resistance ratio of the outer ring up to 73%~83%.