A fan, comprising: a drive structure; an impeller, including a cover plate, the cover plate being installed on the drive structure, the cover plate being recessed in an axial direction of the impeller, and a diameter of the cover plate gradually increasing along an air inlet direction; a wind hood, being covered on the impeller, and an air inlet and an air outlet communicated to the air inlet being provided on the wind hood; and a diffuser, being located at one end of the impeller facing the air outlet, and the diffuser including a first protrusion connected to the drive structure, the first protrusion protruding in a direction of the wind hood, and a diameter of one end of the first protrusion close to the cover plate being smaller than a diameter of one end away from the cover plate, the diffuser also including an inner ring arm connected to the first protrusion and an outer ring arm sleeved outside the inner ring arm, and the outer ring arm being smoothly and transitionally connected to the wind hood, and the first protrusion, the inner ring arm, and the outer ring arm being coaxially arranged; wherein the wind hood, the cover plate, and the diffuser defines a wind tunnel, the wind tunnel includes a variable cross-section portion and an equal cross-section portion, the portion with variable cross-section is arranged close to the air inlet, and the portion with equal cross-section is arranged close to the air outlet; wherein the drive structure comprises a stator assembly, being arranged relative to the air outlet so that the wind flow from the air outlet blowing directly to the stator assembly; wherein an angle between an extension direction of one end of the cover plate facing the first protrusion and an extension direction of one end of the first protrusion facing the cover plate is less than or equal to a first threshold; wherein one end of the first protrusion facing the air inlet is provided with a ring-shaped depression; wherein the impeller further comprises a second blade, being arranged on the cover plate, and located in the wind tunnel, wherein along the air inlet direction, the second blade includes a front edge close to the air inlet and a rear edge close to the air outlet and along the rotation direction of the impeller, the front edge is located in front of the rear edge, characterized in that an angle between an extension direction of the front edge and an axial direction of the drive structure is within a range of 76°±2°, one end of the cover plate facing the diffuser is provided with a ring-shaped protrusion portion located below the cover plate, the depression is in clearance fit with the protrusion portion, and the rear edge is located on a cylindrical surface formed by an outer side wall of the protrusion portion. a drive structure; an impeller, including a cover plate, the cover plate being installed on the drive structure, the cover plate being recessed in an axial direction of the impeller, and a diameter of the cover plate gradually increasing along an air inlet direction; a wind hood, being covered on the impeller, and an air inlet and an air outlet communicated to the air inlet being provided on the wind hood; and a diffuser, being located at one end of the impeller facing the air outlet, and the diffuser including a first protrusion connected to the drive structure, the first protrusion protruding in a direction of the wind hood, and a diameter of one end of the first protrusion close to the cover plate being smaller than a diameter of one end away from the cover plate, the diffuser also including an inner ring arm connected to the first protrusion and an outer ring arm sleeved outside the inner ring arm, and the outer ring arm being smoothly and transitionally connected to the wind hood, and the first protrusion, the inner ring arm, and the outer ring arm being coaxially arranged; wherein the wind hood, the cover plate, and the diffuser defines a wind tunnel, the wind tunnel includes a variable cross-section portion and an equal cross-section portion, the portion with variable cross-section is arranged close to the air inlet, and the portion with equal cross-section is arranged close to the air outlet; wherein the drive structure comprises a stator assembly, being arranged relative to the air outlet so that the wind flow from the air outlet blowing directly to the stator assembly; wherein an angle between an extension direction of one end of the cover plate facing the first protrusion and an extension direction of one end of the first protrusion facing the cover plate is less than or equal to a first threshold; wherein one end of the first protrusion facing the air inlet is provided with a ring-shaped depression; wherein the impeller further comprises a second blade, being arranged on the cover plate, and located in the wind tunnel, wherein along the air inlet direction, the second blade includes a front edge close to the air inlet and a rear edge close to the air outlet and along the rotation direction of the impeller, the front edge is located in front of the rear edge, characterized in that an angle between an extension direction of the front edge and an axial direction of the drive structure is within a range of 76°±2°, one end of the cover plate facing the diffuser is provided with a ring-shaped protrusion portion located below the cover plate, the depression is in clearance fit with the protrusion portion, and the rear edge is located on a cylindrical surface formed by an outer side wall of the protrusion portion.