After form a removal concrete cross ribbed beam floor in a commercial plaza, the vertical cracks are discovered, and the settlement analysis and reinforcement of a Cast-in-situ concrete floor are performed. The detection of the maximum crack is 0.3mm,and the Maximum settlement of cross ribbed beam is 48mm. By theoretical analysis, improper form removal during construction stage makes the actual floor force inconsistent with the design of a load transfer, which is the main reason of the excessive settlement in the cross ribbed beam. Through the economy, ease of handling in construction and other aspects of comparative analyses, the close beam side by epoxy grout and paste the carbon fiber under the beam to reinforce the cross ribbed beam. After reinforcement, no defect is discovered in the project so far.