An accurate understanding of the mechanism of the oxygen evolution reaction (OER) is crucial for catalyst design in the hydrogen energy industry. Despite significant advancements in microscopic pH detection, selective, sensitive, speedy, and reliable detection of local pH gradients near the catalysts during the OER remains elusive. Here, we pioneer an electrochemiluminescence (ECL) method for local pH detection during the OER. For this purpose, a new class of ECL emitters based on ECL resonance energy transfer was theoretically predicted and facilely synthesized by grafting functional fluorescent dyes onto noble 2D carbon nitride. By positioning one of the as-prepared ECL emitters with pH-responsibility neighboring the OER catalysts, local pH gradient generation near the catalysts could be qualitatively measured in real-time with a subsecond resolution. It provided details of the reaction mechanism of the OER and unveiled the catalyst degrading pathway caused by proton accumulation. Besides, the average proton generation rate on the catalyst was also extractable from the local pH measurement as a quantitative descriptor of the OER reaction rate. Owing to the high designability of the grafting method, this study opens up new strategies for studying reaction mechanisms and detecting intermediates.