A foldable baby trailer includes a bottom frame, detachably provided with two rear wheels, two rear frame bars, a handlebar, a top frame bar, two front frame bars, and a trailer bar provided with a front wheel. The handlebar, the top frame bar, the two rear frame bars and the two front frame bars can be directly folded up and closely attached to the bottom frame to reduce the size for storage when the two bottom ends of the handlebar are unlocked from the two rear frame bars. Further, the user can grasp the top grip of the handlebar with the hands to push/pull the foldable baby trailer and to use the foldable baby trailer as an independent baby cart after disconnection of the trailer bar from the bicycle.