A circuit device capable of automatic transmission interface selection and associated method are provided. The circuit device includes a first interface port, a second interface port, a first interface driver module, a second interface driver module, and a buffer module. A first interface driver module receives a first interrupt packet, converts the first interrupt packet into a first command packet, and stores the first command packet into the buffer module. A second interface driver module receives a second interrupt packet, converts the second interrupt packet into a second command packet, and stores the second command package into the buffer module. The format of the first interrupt packet is different from that of the second interrupt packet, while the first and the second command packets comply with a common format.