The invention discloses an automatic rotor folding system for tandem dual-rotor helicopters, which comprises: a vertical shaft, the top of which is fixed with a propeller hub; a plurality of propeller blade assemblies, which are evenly distributed and hinged on the outer peripheral side of the propeller hub; A driving hydraulic cylinder, which is fixedly connected with a plurality of paddle hubs one by one, and the telescopic end of the driving hydraulic cylinder is fixedly connected with the root of the paddle assembly; a hydraulic oil tank, which is fixedly connected with the bottom end of the vertical shaft, and the internal space of the hydraulic oil tank It is a hydraulic oil chamber with an open bottom, and a piston assembly is arranged in the hydraulic oil chamber; a plurality of hydraulic oil delivery pipes, one end of which communicates with the top of the hydraulic oil chamber, and the other end is connected with the cylinder barrel of the driving hydraulic cylinder; the electric push rod , the telescopic end of which abuts against the bottom of the piston assembly. The folding system realizes blade folding through electric push rods, hydraulic oil tanks, and driving hydraulic cylinders. It has a simple structure, low manufacturing cost, fewer electronic control components, and is easy to install and control.