To accomplish precise numerical simulation for transient problem in fluid machinery,a precise method was put forward that numerical simulation based on external performance results.Wherein rotational acceleration was taken into account and experimental results was taken as initial condition and boundary condition of numerical simulation,The merits of sliding mesh technique and dynamic mesh technique were fully applied respectively.The object is a low-specific-speed centrifugal pump,whose impeller is open and designed with straight blades.There is a clear head impact phenomenon.Additional head of model pump mainly comes from rotational acceleration and has nothing to do with flowing acceleration.The rise processes of flowrate and head evidently lag behind that of rotational speed.Pressure difference between pressure side and suction side mainly distributes at middle and exterior regions.With the advance of starting time,the distribution area of pressure difference begins to spread towards center and the difference will augment.Rossby dimensionless number will decrease and back flow region begins to appear.The influence of volute tongue on unsymmetrical flow field will be more and more obvious.