A keyboard input method and an assistant system thereof are provided. In the present method, first, an assistant on-screen keyboard program is launched and an input signal is received. Then, if it is determined that the input signal represents an input character, whether the input character leads to an input error is determined according to an input method of an electronic device. If the input character leads to the input error, a keyboard position of the key corresponding to the input character is obtained, and a partial area of an assistant on-screen keyboard is displayed according to the position, wherein the partial area includes the key corresponding to the input character.