An air conditioner indoor unit, comprising: a housing provided with an air outlet; a volute assembly provided in the housing and comprising an upper volute and a lower volute, an air outlet duct communicating with the air outlet being formed between the upper volute and the lower volute; an air deflector rotatably mounted at the air outlet to open or close the air outlet, a middle portion of at least one end of the air deflector in a length direction of the air deflector being rotatably connected to the housing through a rotation shaft, to make the air deflector have an upward-opening state in which the air deflector is rotated around the rotation shaft to bring a lower side of the air deflector to come close to the upper volute and a downward-opening state in which the air deflector is rotated around the rotation shaft to bring an upper side of the air deflector to come close to the lower volute; a louver assembly connected to a windward surface of the air deflector wherein the air deflector is configured in an arc shape that protrudes towards an outer side of the air outlet wherein a distance S1 between an upper edge of the air outlet and a rotation axis of the air deflector is greater than a distance L1 between an upper edge of the air deflector and the rotation axis of the air deflector, the upper edge of the air deflector being an edge of the air deflector closer to the upper volute than to the lower volute when the air outlet is closed by the air deflector; and wherein a distance S2 between a lower edge of the air outlet and the rotation axis of the air deflector is greater than a distance L2 between a lower edge of the air deflector and the rotation axis of the air deflector, the lower edge of the air deflector being an edge of the air deflector closer to the lower volute of the air outlet than to upper volute when the air outlet is closed by the air deflector; characterised in that: S1 is equal to a sum of L1 and a reserved gap; S2 is equal to a sum of L2 and another reserved gap; and the reserved gap and the another reserved gap are greater than or equal to 3 mm and less than or equal to 5 mm. a housing provided with an air outlet; a volute assembly provided in the housing and comprising an upper volute and a lower volute, an air outlet duct communicating with the air outlet being formed between the upper volute and the lower volute; an air deflector rotatably mounted at the air outlet to open or close the air outlet, a middle portion of at least one end of the air deflector in a length direction of the air deflector being rotatably connected to the housing through a rotation shaft, to make the air deflector have an upward-opening state in which the air deflector is rotated around the rotation shaft to bring a lower side of the air deflector to come close to the upper volute and a downward-opening state in which the air deflector is rotated around the rotation shaft to bring an upper side of the air deflector to come close to the lower volute; a louver assembly connected to a windward surface of the air deflector wherein the air deflector is configured in an arc shape that protrudes towards an outer side of the air outlet wherein a distance S1 between an upper edge of the air outlet and a rotation axis of the air deflector is greater than a distance L1 between an upper edge of the air deflector and the rotation axis of the air deflector, the upper edge of the air deflector being an edge of the air deflector closer to the upper volute than to the lower volute when the air outlet is closed by the air deflector; and wherein a distance S2 between a lower edge of the air outlet and the rotation axis of the air deflector is greater than a distance L2 between a lower edge of the air deflector and the rotation axis of the air deflector, the lower edge of the air deflector being an edge of the air deflector closer to the lower volute of the air outlet than to upper volute when the air outlet is closed by the air deflector; characterised in that: S1 is equal to a sum of L1 and a reserved gap; S2 is equal to a sum of L2 and another reserved gap; and the reserved gap and the another reserved gap are greater than or equal to 3 mm and less than or equal to 5 mm. S1 is equal to a sum of L1 and a reserved gap; S2 is equal to a sum of L2 and another reserved gap; and the reserved gap and the another reserved gap are greater than or equal to 3 mm and less than or equal to 5 mm.