Resulting from the rising levels of atmospheric carbon, ocean acidification has become a global problem. It has significant impacts on the development, survival, growth and physiology of marine organisms. Therefore, a highprecision sensor is urgently needed to measure the pH of sea-water. Iridium wire with a diameter of 0.25 mm is used as the substrate, and an Ir/Ir(OH)_x pH electrode is prepared by a one-step electrochemical method in a Li OH solution at the room temperature. A scanning electron microscope(SEM) observation reveals that it is coated with nanoscale particles. In laboratory tests, the electrode exhibits a very promising pH response, with an ideal Nernst slope(56.14–59.52), fast response, good stability and long life-span in tested pH buffer solutions. For a sea trial,four pH electrodes and one Ag/Ag Cl reference electrode are integrated with a self-made chemical sensor, and a profile detection of nearly 70 m is implemented near Newport Harbor, California on August 3, 2015. The results reflect that the pH value measured by the sensor is very close to the data given by Sea-Bird 911 plus CTD, with a difference value ranging from 0.000 075 to 0.064 719. And the sensor shows a better data matching degree in 0–40m water depth. In addition, the high precision and accuracy of the sensor make it possible to use in the ocean observation field. Resulting from the rising levels of atmospheric carbon. Ocean has a significant impact on the development, survival, growth and physiology of marine organisms. Therefore, a highprecision sensor is urgently needed to measure the pH of sea-water . Iridium wire with a diameter of 0.25 mm is used as the substrate, and an Ir / Ir (OH) _x pH electrode is prepared by a one-step electrochemical method in a LiOH solution at the room temperature. A scanning electron microscope ( SEM) observation shows that it is coated with nanoscale particles. In laboratory tests, the electrode exhibits a very promising pH response, with an ideal Nernst slope (56.14-59.52), fast response, good stability and long life-span in tested pH buffer For a sea trial, four pH electrodes and one Ag / Ag Cl reference electrode are integrated with a self-made chemical sensor, and a profile detection of nearly 70 m is implemented near Newport Harbor, California on August 3, 2 015. The results reflect that the pH value measured by the sensor is very close to the data given by Sea-Bird 911 plus CTD, with a difference value ranging from 0.000 075 to 0.064 719. And the sensor shows a better data matching degree in 0-40m water depth. In addition, the high precision and accuracy of the sensor make it possible to use in the ocean observation field.