Abstract: In order to study the effect law of three-bench construction method on extended tunnel structure and ad? jacent existing tunnel, site monitoring of stress of surrounding rocks and support structure, loosening circle and stress field as well as blast vibration of existing tunnel was conducted based on the extension project of the Houci tunnel on Xiamen-Chengdu expressway in Fujian province. The results show that stress of the rock mass and support structure increases gradually with the time and tends to be stable at last, each bench excavation will cause disturbance and it is represented as sudden increase of stress; after tunnel extension the loosening circle of surrounding rocks at the vault is 6-9 m deep, the loosening circle at the left and right sidewall is 0-6m deep, and the crown settlement is larger than the convergence at the two shoulders, the stress of surrounding rocks of the left sidewall is larger than that of the right sidewall, the stress of surrounding rocks at the depth of 3 m around tunnel is smaller than that at the depth of 6 m, and the excavation induced plastic zone is around 3 m; the actual blast vibration velocity is mostly smaller than the one specified by the designed standard, blasting didn′t cause serious damage to the support structure system of existing tunnel, the max. blast vibration velocity occurs at the place 10 m ahead of the monitoring section and it increases by 2.9%-4.5% compared with that at working face, the better the quality of rock mass, the larger the peak vibration velocity, the attenuation rate of vibration velocity in front of the section with the max. peak vibration velocity is far less smaller than that at the rear.