The invention provides a section-variable flat-hole nozzle. An outlet of the nozzle is annularly distributed in the peripheral direction, the width of the outlet section changes, the width maximum value position and the width minimum value position of the outlet are in counterpoint distribution, and the portion, between the maximum position and the minimum position, of the outlet section is in arc-shaped transition. Compared with the prior art, the design of the flat-hole nozzle with the variable outlet section is adopted, the section-variable flat-hole nozzle has the beneficial effects that sprayed liquid passes the section-variable outlet, and the fluid flow and speed change; and the change of the outlet section is designed reasonably, a liquid film with the uniform thickness can be formed on the surface of a sprayed object by the sprayed liquid, under the condition of the certain mass of the sprayed liquid, the wider liquid film can be formed and covers the target surface, and thus the energy consumption can be reduced, and the ideal infrared stealth effect is achieved.