In order to study the collapse mechanism of deep buried tunnel in mountainous area, taking the Daliangshan No.2 tunnel project of Lexi Expressway as the engineering background, the finite element software MIDAS GTS NX was used to simulate the stability of surrounding rock in the collapse section, the cause of tunnel collapse is analyzed, the corresponding treatment measures are put forward, and the rationality is verified by field monitoring. The results show that this section is located at the junction of argillite stratum and limestone stratum, where joints and fissures develop and rock mass is loose and broken. Seepage of fissure water reduces stability of surrounding rock ,which leads to tunnel collapse. The subsidence of the vault in the collapse section is 177.02mm, and the horizontal convergence of the arch waist is 68.21mm. The surrounding rock deformation is large. The plastic zone of surrounding rock appears on the palm surface of the upper and middle steps, and the maximum strain is 3.027×10-², which means that plastic failure is more likely. The large pipe shed and double-layer advanced small pipe reinforcement support were adopted to reinforce the collapse section, and the treatment effect was good, which provided guidance and reference for the subsequent similar projects.