The invention provides a vacuum coating cavity leak detection system and leak detection method. The system comprises a detected vacuum cavity, a leak detection apparatus, and an electric control system, the lower side of the detected vacuum cavity is connected with the leak detection apparatus, the electric control system is connected with and controls the leak detection apparatus, a sidewall of the detected vacuum cavity is provided with a vacuum breaking device I and a vacuum detection element I, the leak detection apparatus comprises high vacuum pumps, a backing pump set I, a backing pump set II, a vacuum detection element, a vacuum breaking apparatus, a helium mass spectrometer leak detector, and a helium-jetting system, the high vacuum pumps are directly arranged at the sidewall of the detected vacuum cavity through flanges, and the backing pump set I and the backing pump set II are connected in series through a vacuum pipeline and then are connected with the high vacuum pumps in series through a high vacuum butterfly valve I. According to the system and method, an automatic control system is employed, and an electrical component has the one-touch operation function; the sensitivity of leak detection is high, and leakage hole positions can be accurately located, and the size of leakage points can be judged in a quantitative manner; besides, the structure is simple, and the operation is easy.