The invention relates to an indoor unit, an air conditioner and a refrigeration method. The indoor unit comprises a flat plate type refrigerator; the refrigerator comprises a first refrigerant pipe and a first supporting part; the first supporting part is used for being arranged on an indoor roof and provided with multiple first radiating fins; the first refrigerant pipe is arranged on the first supporting part in an curved mode; the inner diameter of the first refrigerant pipe gradually increases from a refrigerant inlet to a refrigerant outlet. By means of the flat plate type refrigerator, the surface area of the refrigerator is increased, the refrigerator is suspended on the roof, cold air descends, and hot air ascends, so that heat-cold circulation is achieved; by means of natural convection, air flow is vertically convective, the flow speed is low, noise is avoided, a human body can hardly feel the air flow, and direct air blowing to the human body is avoided; the inner diameter of the corresponding refrigerant pipe gradually increases with increasing of length so that refrigerant can be gradually evaporated with decreasing of pressure intensity, and uniform refrigeration of all positions of the refrigerator can be achieved.