The invention relates to an adjusting method and a system of a river channel flowing field in a fishway inlet. The system comprises a shunt wall; the horizontal section of the shunt wall is a strip with longitudinal axis as a straight line; a certain angle is formed between the longitudinal axis of the shunt wall and the flowing direction of a river channel; a certain distance is kept from a starting end of the shun wall to a river bank provided with the fishway inlet; and the height of the shunt wall is from the water bottom to above the water surface. A flowing field adjusting facility is adopted to divide a water current flowing field in the fishway inlet into an outer side flowing field and an inner side flowing field; a flowing field suitable for migratory fishes is generated on the inner side flowing field; meanwhile, the outer side flowing field is prevented from disturbing the inner side flowing field; and a high-flowing-speed belt in a shunt inlet is used for stopping the fishes from crossing over the fishway inlet. The system uses the river water current power for adjusting the flowing fields, effectively adjusts local flowing fields of the river channel, is suitable for migration gathering of target fishes, and provides convenience for arrangement of the fishway inlet. The flowing field adjusting system is simple in structure, is low in construction amount, needs no more maintenance, is economical, and is suitable for various hydraulic projects.