Abstract: A new method──filling water preloading method for improving soft clay under an open cais-son foundation is presented.The weight of caisson and water filled in it are used as the load to improveits soft clay substratum during construction period. The method has advantage of improving not theoverburden layer but only its soft clay substratum of the caisson foundation. An elastic-plastic finite ele-ment analysis method to predict the effect of preloading improvement and a field monitoring method areexplained through a case history of caisson foundation of a pump station project.