Based on whether the detector receives a flash signal emitted from surrounding of the thin film sample, the traditional plasma flash method can evaluate whether the thin film is damaged, while it can be easy to confuse plasma flash of air and with that of thin film and make a misjudgment. The elimination of misjudgment on traditional plasma flash method can be realized through comparing the ignition time of the plasma flash of the air and thin film and using the difference of time between them to eliminate this misjudgment. In order to verify the reliability of the new method, a model for calculating the ignition time of air plasma was established by means of the theory of multiphoton absorption and cascade ionization. On the other hand, a calculation model of the same kind was established when the thin film was broken through, and the plasma flash ignition time of air and thin film was calculated to be 1.856 and 7.843 ns respectively by using these established model simulations. The experimental device was set up to update the traditional plasma flash method, and three photodetectors were set up at different positions in the device to collect incident laser signals, air and thin film plasma flash signals, respectively, and the photodetector collecting the incident laser signal was placed on the side of the focusing lens, and the other two detectors were placed around the thin film sample and stay bilateral symmetry, which were used to collect the plasma flash signal of the film and the air, respectively. The signals collected by all photoelectric detectors were converted into electrical signals and sent to oscilloscope synchronously, with the incident laser signal taken as the reference signal, the difference between the initial time of the incident laser signal and the flash signal of air and thin film plasma was the flash ignition time of air and thin film plasma, respectively. When the Nd∶YAG pulse laser, whose pulse width is 10 ns and the wavelength is 1 064 nm, with a radius of 0.015 cm and incident energy of 82.4 mJ laser acts on the sample of single layer Aluminum trioxide film with the optical thickness λ/4 and the diameter of 20 mm. Acquiring various signals under the above laser action, the measured values of plasma flash ignition time for air and thin film after treatment were 2.7 and 7.8 ns respectively. The theoretical calculations and experimental results showed that the ignition time of air is always smaller than that of the film, which is in good agreement with each other. The results showed that the flash of air plasma is earlier than that of thin film when strong laser acts on the surface of single layer Aluminum trioxide film. This difference between the plasma flashes ignition time of air and thin film can be used to accurately identify whether the film is damaged and obtain the criterion to identify if the film is damaged or not . This new method for identifying the flash damage of thin film plasma from time difference provides a technical basis for the elimination of misjudgment in conventional plasma flash method both theoretically and experimentally.