Abstract: The water?flooded rules in Toutunhe reservoir of Shinan?21well block in Junggar basin are greatly influenced by reservoir rhythm. The bottom of positive rhythm zone is heavily water?flooded, the middle?upper part is weakly water?flooded, while the reverse rhythm area is overall heavily water?flooded. In order to find the reservoir area of positive rhythm with less water?flooded risk, the method of interlayer permeability ratio was adopted to confirm the critical value for reservoir rhythm classification, so that determine the positive?rhythm reservoir distribution area. Also, the model for prediction of the weakly water?flooded reservoir thickness was developed to further optimize the well locations and reduce the risk of adjustment. The methods for characterization of reservoir rhythm and prediction of weakly water?flooded reservoir thickness are of high probability and good agreement with the new adjustment wells, and can be applied to other massive sandstone reservoirs with low permeability