A multiband antenna with the broadband function has a radiator, a feed cable, a first extension conductor, and a second extension conductor. The radiator has a microwave substrate, a coupling conductor, a first conductor, a second conductor, a third conductor, and a connecting conductor. The coupling conductor is connected with a positive signal wire of the feed cable. The third conductor is connected with a negative signal of the feed cable for transmitting electrical signals. The radiator generates the multiband mode of the antenna. By connecting the first extension conductor and the second extension conductor with the radiator, the surface current distribution and impedance variation of the antenna can be effectively adjusted to achieve the broadband effect.