The invention discloses a construction process of a bridge crossing an expressway and relates to the technical field of continuous steel box girder bridge construction. The construction process of thebridge crossing the expressway comprises a step of mounting a portal protective frame, wherein the step of mounting the portal protective frame comprises a step of mounting supporting frames, whereinthe supporting frames are correspondingly mounted on the two sides of the width direction of a highway; and a step of mounting a protective frame, wherein the protective frame is erected between thetwo supporting frames, and the protective frame is connected between the two supporting frames. The step of mounting the protective frame is after the step of the mounting the supporting frames. According to the construction process of the bridge crossing the expressway, firstly, the supporting frames are correspondingly mounted on the two sides of the width direction of the expressway, and secondly, the protective frame is erected on the supporting frames, and the protective frame is positioned above the expressway and crosses over the expressway; and therefore, construction personnel can stand on the protective frame to construct a bridge part crossing the expressway so that the influence of blocking the expressway on road passing is avoided and safe construction can also be guaranteed.