In order to study the geological structure characteristics of unconventional tight sandstone and shale reservoir，the relationship between vp/vs and rock lithology，physical properties，poisson ratio，stress and micro cracks of the deep tight sandstone and mud shale was studied based on the data of physical test，mechanical test and acoustic test of Triassic rocks in west Sichuan area. The results showed that vp/vs can distinguish lithology effectively；vp/vs has a positive correlation with porosity and permeability for the formations having the same lithology；abnormally high vp/vs can always occur with the formation of micro cracks under high stress；the rise of vp/vs and the decline of vs both can indicate the development degree of the micro cracks. In the study area，vp/vs is more sensitive to the cracks. vp/vs was characterized and calculated by using poisson ratio and Gassmann equation respectively. It shows that these two methods both have a certain applicability，but the Gassmann equation has a much higher accuracy. A conclusion can be drawn that the theory of elastic wave in the complex formations of tight sandstone and shale is applicable.