The invention relates to a method for observing support cast-in-place beam preloading by utilizing a reflector. The method mainly comprises the steps of fixing the reflector at the position to be observed, inputting an elevation in a measuring instrument, observing and recording coordinates and elevations of the reflector before and after preloading, and calculating the displacement of a support structure according to changes of coordinates and elevations of the reflector before and after preloading. The method has the advantages that settlement observation of support preloading in the large-span high cast-in-place beam construction is achieved; settlement is observed accurately on the premise that constructors are not required to enter preloading fields; the safety of constructors is guaranteed; the measurement is convenient, stationing is easy, places where prisms cannot arrive easily can be observed freely, and the high-altitude operation risks of measuring staff are reduced.