Abstract: The hole enlargement is very serious in the sandstone formation if the hole is exposed to mud for long time. The changes of Young’s modulus、Poisson’s ratio、 cohesion and friction angle of the sandstone are studied by experiments that the cores are immersed in mud for different time. While the stresses around the wellbore are studied, that the changes of Young’s modulus、Poisson’s ratio are considered. Base on these, the model of collapse pressure is established. Thought the experiments and calculation, the conclusions are got as follows: The Young’s modulus，cohesion and friction angle are reduced and the Poisson’s ratio enlarged more as the time that cores are immersed in mud is longer; the stresses are changed with the changes of Young’s modulus、Poisson’s ratio; the collapse pressure is rested with the changed stresses and cohesion and friction angle; the collapse pressure of wellbore will be reduced first and then enlarged.