The invention discloses a method for extending focal depth, including that: Bessel-Gauss radial polarized light passes through an annular diaphragm, so as to modulate amplitude and split the light into two light beams; one light beam is subject to 0-Phi phase coding and then are incident into the same optical beam combining device with the other light beam after optical reflex, so as to form a combined coaxial light beam, and then lens focusing is carried out, thus obtaining the extended focal depth spot. The invention also discloses a device for extending focal depth, comprising a light source generating Bessel-Gauss radial polarized light, an annular diaphragm, a beam splitter, a 0/Phi phase plate, an optical reflex device, an optical beam combining device and an apochromatism focusing lens. In the method and device, Bessel-Gauss laser is taken as a work light beam, the focal depth of extended focal spot is more than 8lambda, the focal spot can be less than diffraction limit of system, and light distribution after extending is more uniform, thus being more beneficial to practical application.