The invention discloses a low-flow-rate harbor basin hydroelectric power generating device capable of achieving free ascending and descending, and relates to the technical field of hydraulic power generation machines. The low-flow-rate harbor basin hydroelectric power generating device capable of achieving free ascending and descending includes a steel pipe pile in a square support frame below anoverhead vertical wharf, a plurality of power arms, a plurality of power generating components and a float bowl. The steel pipe pile passes through the square support frame and is fixed on the river bottom, and the upper end of the steel pipe pile is connected to the upper surface of the square support frame. The float bowl is arranged on the steel pipe pile in a sleeving manner and can float up and down along the steel pipe pile. The multiple power arms are connected to the outer side of the float bowl, and the ends of the power arms are connected to the power generating components. Each power generating component is provided with a power generating element and a propeller. During operation, the power arms and the float bowl float on the water surface under the effect of floating force, the multiple propellers are rotated under the impact effect of low-speed water flow, the power generating elements generate power through kinetic energy of the propellers, electric energy is stored ina power storage box, and the electric energy of the power storage box is used for shore power.