In this paper, a closed hydrostatic drive powertrain (HSDP) composed of an engine, a variable pump, a variable motor, and an energy-efficient adaptive speed-regulating controller (ADC) based on power following is proposed and investigated. The controller can more than guarantee accurate regulation of motor speed through online efficiency estimation based on established loss models of the pump and the motor. It also facilitates the optimal efficiency control of the engine and hydrostatic system through two redundant control freedoms of the HSDP system, making an energy-saving adjustment of the motor speed. At the same time, the controller can prevent engine overload stall and high system pressure by limiting the displacement of the pumps and motors in real time based on the system loads to improve the automatic adaptability of the system to varying loads. Field testing experiments performed by means of a heavy transportation vehicle under different conditions were conducted to verify the efficacy of the proposed controller. The results showed that the average errors of motor speed were 3.3% under empty load conditions and 9.6% under heavy load conditions. In terms of energy saving, comparison tests involving a rule-based controller (RBC) and the ADC were carried out, and the results showed that the energy-saving ratio of the ADC was at least 11.5% and up to 25.8% under empty load conditions and at least 2.8% and up to 9% under heavy load conditions. The ADC controller showed good performance in terms of speed control, load adaptability, and energy saving and a superior advantage due to its simple structure and ease of implementation. Therefore, the proposed controller is an excellent choice for the real-time control of machinery with an HSDP system, especially heavy-duty machinery.