To protect the coastal communities and infrastructures from the devastating damages caused by the extreme ocean surface waves during the severe weather conditions, many coastal defense structures, such as seawall, have been widely constructed along the coastline. In fact, huge ocean surface waves and strong onshore winds coexist in the real harsh coastal environment. Hence, it is of significant necessity to accurately evaluate the overtopping characteristics of coastal defense structures under the combined impact of waves and onshore wind. In this paper, overtopping characteristics of the coastal seawalls under the combined impact of onshore wind and solitary wave are numerically studied. Influences of some main factors, such as onshore wind speed, water depth, wave height, beach slope, and seawall slope, are discussed in detail. The results show that the presence of onshore wind can greatly affect the runup and the amount of overtopping water of seawall under the action of solitary wave. It is hoped that the results of this paper will further deepen our understanding of the overtopping characteristics of seawalls during the severe weather conditions.