A built-in micro-interface papermaking wastewater treatment system and a treatment method are provided in the present invention. The treatment system includes a papermaking wastewater tank, a grid cleaner, an adjustment tank, a centrifugal filter and a sedimentation tank which are connected in sequence, and further includes a heat exchanger, a preheater, a wet oxidation reactor, a gas-liquid separator and a biodegradation tank. A micro-interface unit for dispersing and crushing gas into gas bubbles is disposed inside the wet oxidation reactor. The micro-interface unit includes a pneumatic micro-interface generator, a gas inlet is disposed at a side wall of the wet oxidation reactor, and the gas inlet extends to an interior of the pneumatic micro-interface generator through a pipeline. By arranging the micro-interface unit inside the wet oxidation reactor of the treatment system, the consumption of air or oxygen can be reduced, which realizes low energy consumption and high treatment efficiency.