The present invention provides a circulation-type in-situ high-precision seawater pH value measurement apparatus and a measurement method thereof. The apparatus comprises a fluid circulating device and a signal detection device, wherein the fluid circulating device comprises an indicator bag, a first three-way valve, a second three-way valve, a third three-way valve, a circulating pool, a peristaltic pump, a pulse pump and a fluid pipeline, the second three-way valve, the third three-way valve, the peristaltic pump and the circulating poor form a closed loop when the second three-way valve and the third three-way valve are communicated, the optical path detection device comprises a LED lamp group, a filter group, an optical fiber beam splitter, optical fibers and a photodiode, the light emitted by the LED lamp is filtered through the filter group, the filtered light spreads through the optical fibers and the optical fiber beam splitter, the optical fiber beam splitter exports two beams of light, one beam of the light is imported into the circulating pool through the optical fibers and then enters the photodiode, and the other beam of the light is directly imported into the photodiode through the optical fibers. According to the present invention, the apparatus forms the circulation analysis, and the seawater and the indicator can be completely mixed in the closed loop so as to improve the precision of the measurement system.