An automatic pH value regulating device of a recirculating aquaculture system comprises a water collecting tank of the recirculating aquaculture system, a pH value regulating sustained release device and a filtration device. A pH value detection probe and a spraying pump are arranged in the water collecting tank, the pH value detection probe is connected with a pH value control instrument, and the pH value control instrument is connected with the spraying pump. The pH value regulating sustained release device comprises a sustained release cylinder, spray heads and calcium oxide sustained release blocks, wherein the spray heads are arranged at the top of the sustained release cylinder and are connected with the spraying pump, and the calcium oxide sustained release blocks are arranged on a grating arranged on the lower middle of the sustained release cylinder. The filtration device comprises a filtration cylinder, an inclined tube sedimentation tank and a supernatant liquor overflow launder, the inclined tube sedimentation tank is disposed in the middle of the filtration cylinder, the lower portion of the inclined tube sedimentation tank is a sewage collecting area while the upper portion thereof is a supernatant liquor area, the supernatant liquor overflow launder is communicated with the water collecting tank, and the bottom of the sustained release cylinder is connected with the lower portion of the inclined tube sedimentation tank. The automatic pH value regulating device has the advantages of using an automatic control system to substitute for manual work, more accuracy and convenience in control, real-time continuous control, small pH value fluctuation range and a stable water body.