It is very necessary to learn the stress variation law of primary support of the tunnel with construction, and further find out the causes of "closed" collapse. The loose load has been formed when "closed" collapse happens, and the support structure of steel frame + footlocking anchor bolt + shotcreting is taken to introduce the construction conditions when the collapse happens. Calculation on internal force along with analysis of carrying capacity of primary support is carried out respectively from the perspective of force method of structural mechanics to analyze the 3bench method according to loadstructure model and under the condition of full designed load on primary support. The calculation results show that in the case of total loose of surrounding rock, there is no safety risk in excavation of top heading middle bench, while the excavation of lower bench, especially inverted arch, is with high risk of collapse. The countermeasures are put forward. It is concluded that the support structure may in ultimate stress balance stratus and easily leads to damage and collapse when using steel frame + footlocking anchor bolt + shotcreting mode.