The invention provides a noise reduction device for a fan. The noise reduction device includes an active noise reduction assembly and a passive noise reduction assembly. The passive noise reduction assembly is a hyperboloidal noise reduction cover, the noise reduction cover is located on the air outlet side of the fan in a fan box, an air cavity is formed between the noise reduction cover and thefan box, and the focus of the hyperboloid is located on the air outlet side of the fan. The active noise reduction assembly includes a loudspeaker, a first pickup and an active noise control module. The loudspeaker is located on the air outlet side of the fan. The first pickup is located at the position of the focus of the hyperboloid. The active noise control module is in communication connectionwith the loudspeaker and the first pickup. The noise reduction device can effectively reduce noise under the conditions of meeting ventilation and electromagnetic compatibility design requirements ofa ventilated machine case.