This paper discusses the variation law of factors (metacentric radius,distance from the gravity center to the buoyant center,metacentric height and draft of caisson) impacting the floating stability of the rectangular caisson with the change of the unit weight of concrete and the height of ballast water in the front compartments of the caisson in some port.The results show that if the ballast water in front compartments of the caisson remains the same,the metacentric radius and the distance from the gravity center to the buoyant center will decrease with the increase of unit weight of concrete,but the metacentric height and the draft of caisson will increase.The initial stable equilibrium of the caisson will be changed into unstable equilibrium because of higher water level in the front compartments of the caisson.The risk is not included in the codes and should be considered in design.The decrease of the height of ballast water in the front compartments of the caisson under the premise of satisfying the floating stability is necessary for the construction.