An indoor unit of an air conditioner comprises a panel body (2) encasing side portions and a top portion of the air conditioner, a back housing (1) encasing a rear portion of the air conditioner, and a front panel (3) located in front of the panel body and closely connected to the panel body (2). The back housing (1), the panel body (2) and the front panel (3) define an inner cavity of the air conditioner, in which a heat exchanger, a control element and a ventilation component are arranged. An air outlet (4) is provided at a lower portion of the front panel (3), and an air deflector (5) is provided on the front panel (3) or on the panel body (2) and is configured to open and close the air outlet (4). An air baffle rib (8) is provided at a top portion of the air outlet (4) and extends towards the air outlet (4).