Abstract: This paper aims to predict the performance of 125SII water-jet pump which plans to install in a type of ship,based on the same serial products of 71SII pump by the Similarity Theory and in combination with CFD analys is and experimental data validation.The various performances of 125SII water-jet pump in terms of flow,head,power,efficiency,together with the details reflecting the flow field inside the pump were obtained by CFD anal ysis.The research is to highlight the feasibility of deriving the 125SII geometry model from the 71SII model as to rapid prediction of new design in the conceptual design stage,using the Similarity Theory,in the case o f no design scheme of the pump.For deriving the model,71SII geometry model was enlarged and relevant numerical simulation was performed.The powers at various rotating speed obtained from 125SII model by enlarging 71SII model in geometric proportion and CFD calculation were compared with those given by similarity transformation of the experimental rev-power curve of 71SII water-jet pump.The result shows that the errors are less than 7%.It demonstrates that the numerical simulation of the 125SII geometry model obtained by enlarging the 71SII water-jet pump in geometric proportion is credible.