The invention discloses an experimental system for water and sand movement under a river network convergence region complex riverbed and a simulation method of the experimental system. The system structurally comprises a main stream water tank, a tributary water tank, a water tank downstream area, an upstream A water box, an upstream B water box, a downstream water box, an A water pumping pipeline, sand adders and a B water pumping pipeline, wherein the upstream of the main stream water tank is communicated with the upstream A water box, the upstream of the tributary water tank is communicated with the upstream B water box, the upstream A water box and the downstream water box are communicated with each other through the A water pumping pipeline to achieve water flow circulation, the upstream B water box and the downstream water box are communicated with each other to achieve water flow circulation, the A water pumping pipeline is provided with an A electromagnetic flowmeter, an A valve, and an A variable frequency pump, the B water pumping pipeline is provided with a B electromagnetic flowmeter, a B valve and a B variable frequency pump, the main stream water tank is provided with an A sand adder, and the tributary water tank is provided with a B sand adder. The experimental system for the water and sand movement under the river network convergence region complex riverbed and the simulation method of the experimental system have the advantages that simulation and observation of flow, sediment movement and bed surface form change of complex riverbeds like dells and longitudinal dunes of river network convergence regions are achieved.