Vanadium dioxide (VO2) is an ideal thermochromic material for "smart window" which can reduce energy consumption of air conditioning in modern architectures. However, the application of VO2-based smart window is always limited by the poor luminous transmittance (T-lum), low solar regulation efficiency (Delta T-sol) and unsightly brown color. According to previous researches, compositing has been approved to be an effective method to solve all of above issues for VO2-based smart window simultaneously. In this work, a new thermochromic ionic liquid (nickel-bromine-ionic liquid (or Ni-Br-IL, for short)) was synthesized and introduced to be composited with VO2 nanoparticles for enhanced performance. The composite film demonstrates ultrahigh optical properties: Delta T-sol = 27.0%, which is 1.91 times higher than that of pure VO2 film (14.1%) at a rather high T-lum (T-l,T-lum = 65.9% and T-h,T-lum = 55.3%) with little loss of visible light at high temperature (Delta T-lum = 10.6%). Meanwhile, it can be observed the color change from light yellow to green clearly as the temperature rises. Such enhanced performance will benefit the publicity and application for VO2-based smart window.