The invention discloses a method for protecting a castor fiber cave. A castor fiber nest comprises an underwater outlet an onshore inlet and outlet, and a bionic dam; the bionic dam is built on a river channel close to the castor fiber nest according to surrounding topographic lines, and a still water area is formed on the upstream of the bionic dam; dead wood branches and soil are arranged on thebionic dam, so river water bypasses the arc-shaped bionic dam, and the castor fiber nest is prevented from being impacted; and the length of the bionic dam is determined according to the size of thecastor fiber nest, the width of the bionic dam is larger than or equal to 1.5 m, the height of the bionic dam is 0.5-1.0 m lower than the water surface of the river water, the height of the dead woodbranches is 0.5-1.0 m higher than the water surface of the river water, and the dead wood branches are covered with the soil. According to the ,method, the stability of water level can be guaranteed,a stable aquatic ecological environment for protecting a castor fiber channel and the castor fiber cave is created, the still water area is formed after the bionic dam is built, the castor fiber nestis prevented from being washed away by excessively large and rapid water flow, and damage to the ecological environment due to the fact that castor fibers bite trees excessively to build a dam body isavoided.