A hydro-viscous speed regulating device for heavy-load start of a belt conveyor is provided. An input shaft and an output shaft are connected through a spline tube, an inner friction plate set, and an outer friction plate set. The inner and outer friction plate sets are controlled by an inner oil cylinder and an outer oil cylinder respectively. Oil inlet and oil return of the inner oil cylinder and the outer oil cylinder are controlled by an oil inlet valve core and an oil return valve core automatically. During starting process, pressure oil enters into the outer oil cylinder, and pushes the outer friction plate set to be engaged to provide torque required by the output shaft. After the starting process is completed, the revolving speed of the output shaft rises; the oil inlet valve core and the oil return valve core move outward under the effect of a centrifugal force; the pressure oil enters into the inner oil cylinder and pushes the inner friction plate set to be engaged to provide torque required by the output shaft, and at the same time, oil returns to the outer oil cylinder. That is, during starting process, the outer friction plate set provides large torque required for start, and the inner friction plate set provides small torque during normal operation. The present invention achieves automatic switching between different torque required for start and normal operation. Moreover, the present invention has a compact structure, reliable performance, and low costs, and is applicable in a wide range.