This paper presents a new method for in-situ CO₂ accumulative determina tion, whose operating principle is that, after the reaction of CO₂ to blue alumina adsorbed with thymol phenolphthalein, the activated alumina changes from blue to white in colour. The CO₂-determining tube produced on this principle is buried in the loose layer about 50cm in thickness for 24 hours, then the CO₂ content of soil gas is determined according to the length of discoloured portion of the tube. This method was experimentally used in two areas, which yielded distinct anomalies over the known fault zones.