<p>The preferred method of diving and floating for deep-sea manned submersibles is unpowered. This method has the potential to significantly reduce the energy expenditure of the submersible, extend the operational time of the submersible underwater, and is a fundamental aspect of the submersible’s overall underwater operational capability. Environmental parameters, including seawater density and pressure, as well as the displacement volume of the submersible, fluctuate with depth. This results in a discrepancy between the calculated weights of the submersible diving ballast and floating ballast and the actual requirements. The weight of the diving ballast and floating ballast has a certain effect on the speed of the manned submersible during the processes of diving and floating. Therefore, conducting research into the matching calculation and motion prediction methods for unpowered diving ballast and floating ballast is highly practical for engineering. This paper presents a mathematical model of unpowered diving and floating motion for a manned submersible. It analyzes the forces acting on the submersible and establishes a method for matching ballast and predicting motion during unpowered diving and floating motions in deep water. The feasibility and effectiveness of the method described in this paper are verified and its suitability for engineering applications is demonstrated by comparing and analyzing the sea trial data with those of the “Jiao Long” manned submersible and the “Shen Hai Yong Shi” manned submersible.</p>