A feeding device includes a material pipe, a first container, a second container and a constant-length linkage mechanism. The material pipe is configured for feeding a first material. The first container is configured for receiving the first material and supplying a timed amount of the first material to a first material feed port. The second container is configured for storing a second material and supplying the second material to a second material feed port. The constant-length linkage mechanism is connected to the first container and the second container. When the first container is actually supplying the first material to the first material feed port, the constant-length linkage mechanism allows the second container to supply the second material to the second material feed port simultaneously.