The present invention relates to a method for farming rainbow trout in running water instead of a water tank. The method includes the steps of: letting rainbow trout fry to a specific size to enable the fishes to adapt to an appropriate acclimation salinity level in a freshwater fish farm; acclimating the rainbow trout by moving the rainbow trout to a care in an estuary of a river or a water tunnel path having the salinity of 12 to 25 ‰; and moving the cage to the seat and farming the rainbow trout in the cage for two months to reach the survival rate of 93% or higher.
