Abstract: The fine grid of a township wastewater treatment plant failed to effectively intercept fine plastic debris of 1–5 mm due to its oversized grid gap （5 mm）, resulting in frequent clogging of downstream equipment such as sludge scrapers and mixers, which severely impacted the stable operation of the treatment system. To address this issue, a technical retrofit scheme was proposed from the perspective of mechanical design, which involves replacing the screen with a 2 mm aperture 304 stainless steel punched plate and equipping it with a reclaimed water high-pressure automatic flushing system. Through detailed calculations of flow capacity, head loss, and screen strength, the scheme is confirmed to be technically feasible. A comparative analysis at the equipment level with the scheme of adding a collection device in the grit chamber verifies the rationality and economic efficiency of the retrofit. On this basis, a design method for the selection of fine grids targeting the interception of fine floating debris is summarized. Currently at the stage of technical demonstration, this scheme can provide practical references for the grid equipment upgrade of similar wastewater treatment plants.