The working theory of the Autoclam air permeability test was proposed and key factors to affect the sensitivity were discussed. Results indicate that the sensitivity of Autoclam can be improved using a test ring with a larger internal diameter or a test chamber with a lower volume, while test duration and pressure intensity had no noticeable effect on the air permeability. Test sensitivity can be further improved by reducing the volume of air compressed to the levels which were typically used for assessing normal performance of concrete. It is found that the modified Autoclam is an in situ technique capable of measuring air permeability of HPC.