Abstract: This article studies the difference of sound pressure distribution in a vault with two open ends and in enclosure and long space,by comparing classic Sabine Formula,modified Sabine Formula and ray tracing method's accurate estimation on this kind of space,and at the same time discusses the non-exponential decay curve occurred in that,analyzes the reason of its occurrence,elements and rules of why it has been influenced.Ray tracing method has been used to conduct numerical simulation in this article and results of experiments have been compared with the numerical results to verify the conclusion.The steady sound pressure distribution in the vault with open ends is obviously different from that in enclosure and long space.The classic Sabine Formula can not be used to calculate reverberation time and the estimation of steady sound pressure by ray tracing method is trivial.As for the reason of the occurrence of non-exponential decay curve,it is because the coupling of sound energy between small zones of different "live" degrees within the space and it also depends on if requirements of double-sloped decay curve in coupled volumes theory can be fulfilled or not.If the distance between the observation spot and sound source is not big and both of them are not in the middle of the space,then double-sloped decay curve could be observed very obviously.