Research conclusions：(1) The gas overflow in the Caishenmiao Tunnel was accidental phenomena and gas in the surrounding rock was not in adsorbed state, but in free state, with very low gas content, no pressure, small natural flow of gas and fast attenuation. So the construction area was in low gas area. (2) As the natural and geological environments were complicated, the gas not only overflowed from the gas-bearing stratum, but also probably overflowed from the other stratum occasionally. (3) The gas overflow should be dealt with carefully. It should be known clearly that the hazard of such gaswas small and the gas hazard could be neglected aswell. The some measures should be taken so as to avoid the gas hazard.