Hydrostatic transmission device and engineering vehicle

The invention discloses a hydrostatic transmission device which comprises a variable pump and a motor connected with the variable pump. The motor is a variable displacement motor, and the variable displacement motor is provided with a variable displacement mechanism. The hydrostatic transmission device also comprises a displacement control part for controlling the displacement of the variable displacement motor according to the working load change of a vehicle, and the displacement control part is connected with the variable displacement mechanism. When the working load of the variable displacement motor changes, the position of the variable displacement mechanism of the variable displacement motor can be controlled to change through the displacement control part, thus the displacement ofthe variable displacement motor can be changed to change the output torque of the variable displacement motor. In addition, the hydrostatic transmission device can perform gear shifting while the variable displacement motor p