Construction monitoring plays a vital role in modular construction practice, since faulty installation of individual parts could incur significant delays and extra cost to the project, modular construction eagerly requires a real time progress monitoring. Previous researchers tended to get as-built site data back at first, like making cloud points model or taking photos, and then use it to compare with as-designed BIM model. Such an approach cannot find any potential problem before it happens and will still bring unexpected economic loss and schedule delay. In converse thinking, this paper developed a real time 4D AR system for modular construction progress monitoring, which uses the as-designed AR model to compare with the as-built site, in which a markerless AR registration method is established to link the 4D BIM data with the as-built ones directly. In addition, mock up components are used as a scale model to validate the system. The result indicated that, the system can facilitate users to avoid possible errors by giving helpful assembly sequence monitoring and powerful tool for schedule control.