The low embankment of the expressway is limited by the height of subgrade, and some special problems such as excessive settlement or uneven settlement are easily caused after construction. At present, the subgrade design and maintenance codes do not uniformly regulate the design of the low embankment, resulting in many problems that would affect the normal use of the structure during the construction and operation of the low embankment. Based on the risk assessment theory of geological disaster, this paper analyzed the effect of some factors such as drying and wetting cycle as well as traffic load on the low embankment of the expressway. In addition, considering the maintenance and repair, these factors were modified and the evaluation method of low-embankment-subgrade in service status was proposed. Combined with fuzzy comprehensive evaluation method and matter-element analysis method, the structural quality vulnerability in construction period, the risk of subgrade in operation period and the modified value of maintenance and repair were analyzed. And then the comprehensive risk of low-embankment-subgrade was obtained. The method was applied to a low embankment project in northern Zhejiang. The result shows that the comprehensive risk is low and the service state risk is basically consistent with the filed survey result, which illustrates the feasibility of the proposed evaluation method.