A retractable post with an alarm device includes a plurality of retractable pipes retractably sleeved with each other. A retractable air whistle is secured by a latch block at a proper location inside the retractable pipe. A plug piece is installed at a bottom of the retractable post. The plug piece and the latch block make an interior of the retractable pipe present a dummy seal state, wherein the retractable pipe is almost but not completely sealed. When an abnormal rapid falling displacement of the retractable pipes occurs, the latch block and the retractable air whistle compress the air inside the retractable pipe, and then a bellow of the retractable air whistle is further compressed, such that the air inside the bellow is rapidly discharged through a sounding valve disposed at a front end of the air whistle, thereby generating an alarm sound.