Abstract: A multi-branch structure is proposed to converge ultrasonic plane waves to a focus at sub-wavelength scale. The structure is designed to be constituted with integral length of apertures, which allows the Fabry-Perot (FP) to be exhibited in all apertures simultaneously, then the generated evanescent waves containing sub-wavelength detail are amplified effectively and the sub-wavelength focusing is achieved. Futhermore, addition of surface periodic is demonstrated to improve the transmission gain and focusing resolution at certain frequency. The designed structure can achieve sub-wavelength focusing in a wide frequency range with enhanced transmission gain. This work will have potential applications in the ultrasound imaging and treatment.