Abstract：Fish feeding is a key link to the pre-processing of fishes, as the recent increase in the reproductive output offreshwater fishes.At present,mechanical processingis widely used in the pre-processingof freshwater fishes. However, most fish pre-treatment equipment is lacking of the specific device to automatically position the fish body when fish feeding, rather than manually feeding. The current manual feeding is restricting the industrial development of the pre-treatment processing for freshwater fishes due to its high labor intensity and low efficiency when fish feeding. To realize the automatic feeding of freshwater, this present study aimed to design a sort of mechanical orientation device for pre-treatment processing equipment when fish feeding. A typical bulk freshwater fish, Carassius auratus,was selected as research objects. This selection is because Carassiusaur atushas approximately wedge-shape abdomen in the cross-sectional fish body, where exhibit certain mechanical characteristics when the fish's abdomen and back were squeezed at the same time. Asuitable method was then proposed for the orientation of the freshwaterfish'sabdomen and back, as well the related mechanism was analyzed.A test device was finally designed and manufactured for the orientationof the freshwater fish'sabdomen and back. The systematic experiments were carried out on the test device for the freshwater fish'sabdomen and back when fish feeding. The orientation success rate and orientation time were recoded during the test,to be used as the evaluation index of each test. The main parameters of the test device were optimized, and the optimal values were also determined to locatethe freshwater fish'sabdomen and back. The main working parameters of the test device were set as the roller shape, roller gap, fish bodytransport speed and the roller rotation rate. First, singlefactor tests were utilized to investigate the effects of these parameters on the abdomen and back orientation of the fish body. Then, an orthogonal test was performed to systematically explore the specific factors on the position operation of the fish body. The results of the orthogonal test showed that the combination among various factors in the optimal level was: the gap of the roller was 50mm, the rotation rate of the roller was 50 r/min, and the transport speed of the fishbody was 0.6 m/s.Under this optimal level combination, the orientation success rate was 100%, orientation time was 0.55 second, indicating that the working state of the test device for freshwater fishes'abdomen and back was reached the optimal state. The related results on the abdomen and back orientation of freshwater fishes can provide insightful theoretical basis and sound technical reference for the development of orientation equipment for feeding in freshwater fish processing.