In theory the disturbance generated by the ETKF (Ensemble Transform Ka lman filter) method has the same probability in the observation space, so it can complement the shortage caused by the ensemble generation scheme Breeding. Based on ETKF theory， a simple mesoscale ensemble system is built usin g the GRAPESMeso model and a case of rainfall occurred at 8th to 12th Nov 2005 is studied in order to verify whether such method can be applied into an i mperfect timevarying regional ensemble model with limited ensemble member. The experiment s indicate that this method can be used into the regional ensemble system，and it also can maintenance its priority, and the experimental shortcomin gs and the study focus in the future are also indicated.