Abstract: Blending hydrogen into natural gas in a certain proportion and transporting it through the existing natural gas pipeline network to end-users for combustion application is regarded as one of the potential ways to use hydrogen on a large scale at a low cost. However, the variation of the hydrogen blending ratio will affect the operating parameters of combustion devices. As a result, the flashback characteristics will change, threatening the operational safety. In the present study, a parameter β was defined to quantitatively compare the flow rate of the fresh premixed gas and the flame speed to characterize the risk of flashback. Verified using experimental data, the risk of flashback significantly increased when the β value was lower than 0.6. Moreover, results showed that flashback was more likely to occur in domestic appliances (0.2 < β < 1) than in gas boilers (0.95 < β < 1.4), and the available hydrogen blending ratio to maintain the safety of the former was lower.