The invention provides a composite electrode type pH sensor based on a CMOS (Complementary Metal Oxide Semiconductor) process and a preparation method of the pH sensor. The sensor comprises a substrate, a sensing detection area, a working electrode, a reference electrode, an electrode lead and a bonding pad. The working electrode is arranged above the sensing detection area, and the reference electrode is positioned inside the sensing detection area. The preparation method comprises the following steps: manufacturing a substrate from an alumina ceramics material; manufacturing a semiconductor substrate and a well region; manufacturing the reference electrode; manufacturing an isolation region and an electrode oxide layer; manufacturing a metal aluminum layer; manufacturing the working electrode; manufacturing the electrode lead; and manufacturing the bonding pad. The CMOS technology is adopted, and the reference electrode is manufactured inside the device, so that reference electrode is prevented from being interfered and influenced by an outside environment, and the stability of the reference electrode is improved. The sensor has the characteristics of high stability, high sensitivity, high repeatability and the like, is easy to integrate and miniaturize and suitable for in-situ measurement of pH of a soil cultivation matrix and other heterogeneous systems, and solves the pH measurement problem in case of less moisture content.