The invention discloses a filtering device of an aquarium. The problem that when water circulates to be filtered, the normal running of an aquarium is influenced if a filtering device is replaced is solved. A left reversing valve is arranged at the joint of a water inlet pipe and a bypass pipe. A right reversing valve is arranged at the joint of a water outlet pipe and the bypass pipe. The left reversing valve and the right reversing valve are linked and have a first position at which the water inlet pipe and the water outlet pipe are communicated with a filtering barrel, and a second position at which the water inlet pipe and the water outlet pipe are communicated with the bypass pipe. At the working state, water sequentially passes through a water pump, the water inlet pipe, the filtering barrel, the water outlet pipe and the aquarium; when the filtering barrel needs to be replaced, the left reversing valve and the right reversing valve are adjusted, and water sequentially passes through the water pump, the water inlet pipe, the bypass pipe, the water outlet pipe and the aquarium; by adjusting the left reversing valve and the right reversing valve, the flowing direction of water flow is changed in the filtering circulating process, the aquarium can normally run even when water does not pass through the filtering device, and the filtering device can be conveniently replaced.