In this paper, an ultrathin metalens has been proposed based on a holographic metasurface that consists of elongated apertures in 40 nm gold film, which exhibit intriguing properties such as on- and off-axis focusing and also can concentrate light into multiple, discrete spots for circularly polarized incident lights. First, the spatial transmission phase distributions of the designed metalens with arbitrary focusing can be obtained by computer-generated holography. Then, the discrete phase distributions can be continuously encoded by subwavelength nanoapertures with spatially varying orientations in gold film. The simulation results show that our designed metalens can work efficiently for different types of focusing. Finally, our metasurface shows superior broadband characteristics between 670 and 810 nm, and the corresponding focal lengths of the designed lenses also can be efficiently modulated with the incident lights at different wavelengths.