Abstract: Mechanized recycling of used plastic film has become an important and effective method to control the pollution of 'white pollution' in Xinjiang cotton fields. The working feature of the plastic film recovery machine is to recycle the used plastic film together with impurities such as cotton stalks, cotton bolls, cotton husks, and soil. The mixtures are not easy to separate due to the entanglement between the used plastic film and the cotton stalk and the lack of effective separation methods and equipment. This research proposed a solution to separate solid-phase plastic film and solid-phase impurities by using liquid water as the medium. Basing on studying material characteristics and theoretical analysis of water washing separation, taking the relaxation time difference and the maximum vertical displacement difference between the plastic film and cotton stalks as the response index, the single factor experiment of flow field characteristics and material properties was carried out by using the controlled variable method on a self-designed testbed of suspension separation. The significant factors affecting the separation between the plastic film and cotton stalks were determined as the speed of the impeller, area of plastic film, and aspect ratio of cotton stalks. Furthermore, a three-factor five-level quadratic rotation orthogonal combination experiment was carried out, and a multiple mathematical regression between the influencing factors and the response index was established by data optimization software Design-Expert V10.0.4. The best parameter combination including the speed of the impeller 202 r/min, area of plastic film 1 271.4 mm2, the aspect ratio of cotton stalks 13, was obtained by using the software optimization function, corresponding relaxation time difference and maximum vertical displacement difference between the plastic film and cotton stalks optimization values were 4.20 s and 22.1 cm. It was verified by experiments that the average values of the relaxation time difference and the maximum vertical displacement difference were 4.34 s and 21.2 cm respectively, and the relative errors from the predicted value were 3.23% and 4.25%, which proved that the established model and the optimal parameters after optimization were feasible. Through the post-processing of the video frame by frame, the sinking process and upward gathering behavior of the cotton stalk and plastic film were studied. The behavior of settlement and aggregation of plastic film and cotton stalk was a process of mutual conversion between the horizontal stable orientation and the vertical movement orientation. And their trajectories were a set of concentric circles centered on the axis of the impeller. Compared with cotton stalks, the plastic film had a longer response period to the force of the water medium, and it had an obvious flipping phenomenon in the three-dimensional space. This research was an attempt and exploration of new schemes for the treatment of residual film pollution in Xinjiang cotton fields. By referring to the relevant theories in the field of solid-liquid multiphase flow, it was carried out the repeatable experimental research of the flow field and materials quantitatively. Then it was carried out a qualitative analysis of the movement process of the cotton stalk and plastic film. Subsequent research could combine residual film recovery machine agricultural machinery with cotton field irrigation agronomy, such as by installing mechanical devices in the surface diversion channel of agricultural irrigation water to complete the cleaning, separation, and recycling of the mechanically harvested film in one operation. Results of this study could provide a theoretical and scientific basis for the further research of cotton stalks trash separation technology and could also provide a reference for the optimization of straw crushing technology and film shredding technology parameters.