Real-time monitoring of extracellular pH (pHe) at the single-cell level is critical for elucidating the mechanisms of disease development and investigating drug effects, with particular importance in cancer cells. However, there are still some challenges for analyzing and measuring pHe due to the strong heterogeneity of cancer cells. Thus, it is necessary to develop a reliable method with good selectivity, reproducibility, and stability for achieving the pHe heterogeneity of cancer cells. In this paper, we report a high-throughput, real-time measuring technique based on polyaniline (PANI) microelectrode arrays for monitoring single-cell pHe. The PANI microelectrode array not only has a high sensitivity (57.22 mV/pH) ranging from pH 6.0 to 7.6 but also exhibits a high reliability (after washing, the PANI film was still smooth, dense, and with a sensitivity of 55.9 mV/pH). Our results demonstrated that the pHe of the cancer cell region is lower than that of the surrounding blank region, and pHe changes of different cancer cells exhibit significant cellular heterogeneity during cellular respiration and drug stimulation processes.