A gas-mixing device includes: a shell, where the shell is provided with an air channel for conveying air, a fuel gas channel for conveying fuel gas, and a mixing-gas channel connecting downstream of the air channel and the fuel gas channel. The fuel gas channel includes a first sectional flow area and the air channel includes a second section flow area; which are located on the movable part of the shell. The movable part is provided with a flexible separation component which hermetically separates the air channel and the fuel gas channel. The movable part penetrates through the flexible separation component and enters the air channel and the fuel gas channel and performs a movement to change the first and second sectional flow area.