The subway foundation pit engineering is an engineering of high risk, it is great significance to evaluate the risk scientifically and accurately for making the security decision. Therefore, in this study a risk assessment approach based variable weights and matter-element extension was put forward. Firstly, the matter-element extension model was built. Secondly, calculate the degree of closeness of the matter element and determine the risk level according to the degree of closeness. Furthermore, the sensitivity analysis has been made through the variable weights. Finally, with engineering concrete example, this paper analyzed the risk of the pit body structure supported by the underground continuous wall and points out the sensitivity of indicators. The results show that the evaluation method can be more widely used in engineering than other evaluation methods.