The invention relates to a variable-size brush comprising a fixed handle, a movable rod, a spring and a brush head; the rear end of the deformable hollow brush head made of soft rubber material is fixed in the front end of the fixed handle; the movable rod penetrates the fixed handle from back to front and slide back and forth along the fixed handle; the movable rod front end extends outside the fixed handle front end, and is located in the brush head; the movable rod front end is connected to the inner side of the brush head front end; the spring is sleeved on the front section of the movable rod; the spring front end is fixed with the movable rod front end; the spring rear end is fixed with the fixed handle front end. The invention also relates to a variable-size brush head changing method; the variable-size brush is simple and reasonable in structure, low in cost requirements, concise in moulding, safe and easy to use, can realize single-hand operation, and belongs to the brush technical field.