A metalens is the flat and ultrathin surface made of billions of sub-wavelength elements and can bend light like traditional optical lens, which make it attract enormous interests. However, two fundamental issues need to be addressed before metalens can replace traditional lens. First, a large diameter (up-to-cm) metalens is extremely difficult to simulate and optimize, due to the large area, lack of periodicity, and multiple parameters on billions of sub-wavelength elements. Second, to obtain a high-quality image within certain distance, a variable focus length is highly desired in most of modern optical system. In this work, we develop an analytical model based on an optical phased array antenna with the focusing phase profile, and accurately predict the far field radiation pattern for a large-area metalens with significant low computational cost. The beam-width of system and depth-of-focus (DOF) are given with respect to wavelength, element spacing and aperture size. To realize the focus tuning function on silicon metalens, the cascaded PIN junction phase shifters enhanced by Fabry-Perrot cavity are attached to metalens to enable the 2π phase variation. At last, a silicon metalens with a diameter of 400um and a focus of 93um, at 1.55um wavelength is verified in FDTD simulation. The results show that the beam-width, DOF and focus tuning range agree well with the analytical model result. The f/10 axial displacement is achieved with a carrier injection of 1019 cm-3. This focus tuning mechanism could be deployed to many attractive near-infrared applications, such as fluorescence microscopy and LIDAR.