Due to the large-span and flat-section of the four-lane tunnel, which have many construction sections and complicated construction procedures, the lining structure and surrounding rock have been disturbed for many times, resulting in significant differences in mechanical characteristics to the small-span tunnel. In this paper, by using the software of finite element analysis, based on Qingchuan tunnel of renzun expressway, the construction process of shallow buried partial pressure section of the entrance are simulated, and the settlement of tunnel vault, horizontal convergence and distribution and development of plastic zone of surrounding rock are studied, by summarizing and analyzing the calculation results, the construction mechanical response characteristics of the shallow buried bias section at the entrance of the four-lane tunnel are obtained, and the corresponding reasonable construction optimization measures are proposed according to the different characteristics, so as to improve the construction safety and tunnel engineering quality. The research methods and results can provide guidance and reference for similar tunnel engineering design and construction.