Mobile cranes are widely applied in the construction industry all over the world. As a central component performing lifting operations and maneuvering heavy loads over long spans, mobile cranes are associated with a large number of accidents on construction sites. Typical mobile crane-related accidents include electrocution, crane tip over, and struck by crane parts or load. One of the main reasons behind these accidents is due to the operator's lack of visibility of the crane itself and the surrounding environment. In this context, visual monitoring and alarming is an effective way to identify and mitigate potential mobile crane-related hazards on construction sites. With the goal of providing real-time safety assistance for mobile crane operations, this paper presents a cyber physical systems (CPS)-based approach to monitoring mobile crane operations in real-time and providing enriched information to the crane operator. The developed CPS-based system was validated on a full-scale construction site to demonstrate its practical functionality. Test results indicate that the developed system is effective in reconstructing crane motions in real-time and providing pro-active control feedbacks to the crane operator.