The invention discloses a device for replacing a valve at a wellhead of a water injection well and belongs to the technical field of water injection in an oil field. The device comprises a screw rod, a sealing sleeve, an ejection sleeve, an ejection nut and an elastic expanding sleeve, wherein the excircle of one end of the screw rod bulges to form a first stop part, and the elastic expanding sleeve sleeves a rod body of the screw rod and is contacted with the first stop part; an excircle of one end of the ejection sleeve bulges to form a second stop part, the ejection sleeve sleeves the rod body of the screw rod, and one end surface of the second stop part is contacted with the elastic expanding sleeve; the sealing sleeve sleeves the ejection sleeve, the ejection nut is screwed on the rod body of the screw rod, and one end surface of the ejection nut is contacted with one end surface of the ejection sleeve. The valve at the wellhead of the water injection well can be replaced under pressure.