Accurate focus positioning of a laser beam is an important basis for improving laser engraving, cutting and welding accuracy, but the traditional methods are not fit for automatically finding high-power laser focuses. The plasma caused by laser ablating metal is able to emit a large amount of ultraviolet radiation. Therefore, in this paper a focus automatic positioning method for high-power infrared lasers is proposed and the relevant device is fabricated with a GaN Schottky photodiode as sensor and a 304 stainless-steel laser target as consumable material. As a contrast experiment, the depths of the laser ablation pits on the stainless-steel target are measured with a confocal microscope to find the deepest pits. The focal position difference is 24 μm between the two positioning methods, when the experiments using a 1064-nm-wavelength fiber laser engraving machine with a pulse width of 100 ns, a repetition frequency of 20 kHz and an average power of 10 W.