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The effect of the backpressure on the plastic deformation of the billet material was investigated by finite element simulation in the equal channel angular pressing/extrusion (ECAP) process. The multiple-pass ECAP experiment results were also analyzed. The results show that the degree and uniformity of plastic deformation can be effectively enhanced by backpressure application during ECAP for every pass. In the multiple-pass ECAP experiment, there is a decrease in the equilibrium grain size and the processing temperature in the ECAP process with backpressure.