Abstract: Monitoring observation localization for Geheyan Proj ect in 1998 flood season was the deep outlets,the surface spillway,the still ba sin and auxiliary energy dissipators.The observation content included the flow p attern,time average pressures ,flow velocities,fog rain and inspection in situ.T he monitoring results indicated that the hydraulic properties of every item of t he spillway,deep outlets,stilling basin and auxiliary energy dispersion are overall good;on the flow boundaries of floodreleasing structures no cavitation erosio n occurred;the two side slopes of stilling basin stage 1,the energy dissipation baffles and diffe rential sills on the stilling basin stage 2 were elementarily perfect.But it is found from an inspection after dewatering in February,1998,that there was p artial wear in the stilling basin,and partial collapse at the protection bank of dividing dike of left bank produced in the releasing flood process of 1998.In v irtue of releasing flood by using skijump type structure,fog phenomenon is mor e serious ,therefor appropriate meaures must be adopted to secure safety operat ion.Through comparison of data observed in prototype ,measured on model and obta ined by computation,some basical agreements in general trend and among correspon ding values therefrom are achieved. 