A stepless adjustable telescopic device includes a first support, a second support, and a folding arm connecting the first support to the second support. The folding arm includes a first quadrangular structure, a second quadrangular structure, and a power support device connected to the first quadrangular structure. A first end of the first quadrangular structure is rotatably connected to the first support. A first end of the second quadrangular structure is rotatably connected to the second support, and a second end of the second quadrangular structure is rotatably connected to a second end of the first quadrangular structure via a rotary connection structure. The power support device and the first quadrangular structure form a triangular structure, and the power support device is configured to drive the first quadrangular structure to rotate such that the first quadrangular structure drives the second quadrangular structure to rotate synchronously with the first quadrangular structure.