Abstract: The exploration of the very far space in solar system will be a main target of the next phase of deep space exploration activities. The achievement of this goal relies on the breakthrough of the continuous propulsion power technology. Basing on the description of the Stuhlinger's solution for spaceflight under continuous constant thrust, the relationships between the key parameters such as the payload ratio, mission time and flight distance of the very far space missions and the engine performance are analyzed. The conditions that the engine specific impulse and power need to satisfy and their effects on the payload ratio and the final flight speed are given under specific mission duration and specific flight distance requirements. In addition, the large elliptical transfer orbit and its transfer energy requirement of planetary exploration in solar system are derived under the two-body orbital dynamics, and the key technical indicators involved in the continuous thrust solar sail mission are theoretically calculated. These conclusions are the summerized by the basic theory of continuous propulsion scheme in deep space exploration, which can provide important inspiration and guidance for China's future deep space activities.