The invention provides a system and method for pressurizing a vehicle LNG gas cylinder with CNG. A CNG storage tank is additionally arranged on the basis of the LNG gas cylinder, and compressed high-pressure natural gas in the CNG storage tank is adopted for filling gas and increasing pressure into the LNG gas cylinder after pressure reduction. Compared with a traditional LNG air-temperature-type self-pressurization mode, the influence on the environment temperature by a pressurization system is eliminated, and the pressurization speed of the LNG gas cylinder is increased. Meanwhile, the condition that pressurization cannot be achieved when the LNG gas cylinder lacks of liquid is avoided, great potential risks existing in a traditional exposed self-pressurization finned coil pipe are also eliminated, and heat leakage points of the LNG gas cylinder are reduced. A system pressurizing loop pressurizes a CNG gas cylinder in the way that a CNG overflowing valve, a CNG pressurization stop valve, a pressure reducer, a CNG pressure increasing and regulating valve and a gas return stop valve sequentially communicate with a gas-phase space of the LNG gas cylinder. The CNG sequentially communicates with the CNG gas cylinder for supplementing and adding gas through a gas source, a CNG adding opening, a CNG inlet ball valve and a CNG inlet check valve.