Thin-film lithium-niobate-on-insulator (LNOI) is a promising platform for nonlinear optics and optical interconnect. It is critical to realize high efficiency for coupling optical signal into lithium niobate photonic integrated circuits. Here we demonstrate a low-loss edge coupler for coupling between a standard optical fiber and sub-micrometer LN strip waveguides. The coupler consists of an inverse tapered mode size converter, as well as a tapered SU-8 polymer curved waveguide which functions an interface between the single-mode optical fiber and the mode size converter. The simulation shows that coupling efficiencies for TE mode and TM mode are -0.42 dB and -0.75 dB at 1550nm, respectively. The coupling efficiency is higher than -0.77 dB for both TE and TM mode in wavelength range of 1450 nm to 1650 nm. This means that our edge coupler has polarization insensitivity and ultra-wide bandwidth.