Numerical simulation and test forming of the isothermal precision forging of an impeller was carried out. The forming processes were simulated with DEFORM-3D to obtain the forming characteristics and metal flow pattern. It indicated that the impeller can be forged by the method of isothermal-forging, and the deforming process can be divided into 3 stages. The forming of blades was depended on the extrusion of materials. During the forming, uneven deformation was existed in the billet, especially in the field of the blade root. The forging load was increased rapidly during the later stage of the forming process, and the maximum forging load was about 2961kN. According to the simulations, the die structure and the billet dimension were determined, the forging die was designed and manufactured, and the precision forgings of the impeller were produced successfully. Both of the simulation and the test forming indicated that the impeller forging could be performed with the assembled structure die and the isothermal extruding forming style satisfactorily. The ideal parameters to produce the precision forgings of the impeller are: a forging temperature of 450°C and a punch speed of 1mm/s. Under these conditions, the forgings of the impeller can be produced with full blades, smooth outer surface and good flow line, which can meet the requirements of the precision forging of impellers.