The invention provides a wall-mounted air conditioner indoor unit which comprises a casing and a flow guide piece; a first air supply outlet and an air duct connected with the first air supply outletare defined in the casing, and the inner wall, close to the first air supply outlet, of the air duct is gradually reduced such that the flow cross section of the air duct is gradually reduced in the airflow direction; the flow guide piece is arranged in the air duct, and an air outlet gap is defined between the flow guide piece and the inner wall of the air duct so that the air supply airflow blown to the first air supply outlet is guided to the inner wall of the air duct, and the air supply airflow gradually flows out of the first air supply outlet towards the airflow center in a gathered mode under the guiding effect of the gradually-reduced part of the inner wall of the air duct; the flow guide piece is of a hollow structure, and spray holes are formed in the side, facing the first airsupply outlet, of the flow guide piece, and the flow guide piece is configured to produce water mist; and in the flowing process of the supply airflow, a negative pressure environment is formed at thespray holes, so that water mist is mixed into the supply airflow and blown out of the first air supply outlet together. The wall-mounted air conditioner indoor unit has better effects of long-distance air supply and strong air supply.