The utility model relates to an air compressor, in particular to a gear type air compressor for a PTA (pure terephthalic acid) device. The gear type air compressor is provided with three gear shafts, two ends of the middle gear shaft are respectively connected with a driving machine, the middle gear shaft is a driving shaft, a gear on the middle gear shaft is respectively meshed with gears on the gear shafts on two sides so as to drive the driven gear shafts on the two sides to rotate, impellers are respectively disposed at two ends of each of the driven gear shafts on the two sides, a volute casing assembly mounted on a gear box is covered outside each impeller, each stage of volute casing assembly is gripped on the gear box via a flange, and the volute casing assemblies are connected with a gas cooler via inter-stage air pipelines. The PTA device adopts the three-dimensional semi-open type mixed-flow impellers for the first time, thereby ensuring excellent pneumatic characteristic of the total air compressor. An inlet blade guiding adjustor is arranged in front of the first-stage impeller, thereby realizing high-efficient energy conservation adjustment.