A foldable safety handrail assembly includes a mounting bracket, a fixing sleeve mounted on the mounting bracket, a release sleeve mounted on the mounting bracket and having a release groove, a rail having a pivot tube which has a first locking groove and a second locking groove, a locking tube extending through the fixing sleeve, the pivot tube and the release sleeve and having a limit slot, a locking member mounted in the limit slot and detachably locked in the first locking groove or the second locking groove, a control member mounted in the locking tube to move the locking member, and a push knob mounted on and pushing the control member. Thus, the user only needs to push the push knob to unlock and fold the rail so that the rail can be unlocked and folded easily and quickly.