The invention discloses a central diffusion type tiny-oil ignition combustor. An inner primary air pipe is mounted in a main primary air pipe, one section of the main primary air pipe extends into a tertiary pulverized coal combustion chamber, another section of the main primary air pipe is mounted in the inner primary air pipe, a pulverized coal concentration cone and a vortex-controllable flameexpanding blunt body are sequentially mounted in the inner primary air pipe along a flowing direction of fluid, and are coaxially installed; a small section of an L-shaped ignition oil gun in an abrasion-resistant sleeve is aligned with a cone of the vortex-controllable flame expanding blunt body, a small section of an L-shaped air inlet pipe is aligned with the bottom end of the vortex-controllable flame expanding blunt body in a hollow structure, air outlet holes are circumferentially uniformly distributed at the bottom end of the vortex-controllable flame expanding blunt body, and a plurality of circular secondary air inlet holes are equidistantly arranged on a closed end surface of the tertiary pulverized coal combustion chamber. High-temperature flame ejected by the ignition oil gun is ejected onto the flame expanding blunt body, and is obliquely diffused along the outer edge of the blunt body to a wall surface of a round pipe, ignition around a ring surface is realized, and ignition efficiency is greatly improved. In addition, the vortex-controllable flame expanding blunt body can control the size of a high-temperature backflow area formed at an outlet of a primary combustion chamber, so that fuel adaptability can be enhanced.