In the thixo-extrusion process, the fiber orientation can be effectively controlled by applying the hydrostatic pressure (i.e., backpressure), and consequently improving mechanical properties of short fiber reinforced composites. In this paper, the variation regularities of the deformation field and fiber orientation during the thixo-extrusion process of Csf/AZ91D composites with and without backpressure were investigated experimentally. The equivalent strain rates along the flow line were obtained based on the flow function method. By employing the image processing technology, the angle of fiber orientation after extrusion was statistically analyzed. The results show that there was an obvious equivalent strain rate gradient from the outer wall to the core of the composite without applying backpressure. At the same position, the fiber orientation angle was almost proportional to the equivalent strain rate. Furthermore, the applied backpressure enhanced the ability of the composite to fill the outer corners of the mold, resulting in a smaller equivalent strain rate gradient across the cross-section. Comparing with the thixo-extrusion without backpressure, the equivalent strain rate distribution is more uniform and fiber orientation angle is more consistent by the application of backpressure. The research results have important guiding significance for regulating the plastic deformation of Csf/AZ91D composites.