A charging system includes a transmission line, a power adaptor, and an electronic device. The transmission line includes a first terminal and a second terminal. The power adaptor is coupled to the first terminal and generates a charging voltage and a first voltage signal. The charging voltage is at a first voltage level. The electronic device is coupled to the second terminal and receives the first voltage signal. When the first voltage signal is larger than a first preset voltage signal, the electronic device outputs a second voltage signal to the power adaptor. When the second voltage signal is larger than a second preset voltage signal, the power adaptor adjusts the charging voltage into a second voltage level.