To obtain the time windows of reliable effect for explosive null gate under the different gap thickness, the numerical simulation and the experimental research of improved explosive null gate with round air groove of charge size of 0.6 mm×0.6 mm and gap thickness of 0.6-1.1 mm were carried out through LS-DYNA software and test. The numerical simulation results show that when the gap thickness is less than 1.0mm, the closing time of explosive null gate dose not exceed 0.9 μs. However, with the further increase of the gap thickness, the success probability of explosive null gate will be significantly reduced. The explosive null gate fails even at 1.1 mm. The test results and simulation ones have a consistent regularity. When the gap thickness reaches 1.0 mm, the reliability time window of explosive null gate is [4.0 μs, + ∞). When the gap thickness is 1.1 mm, the success probability of explosive null gate is low.