A frame structure for foldable baby trailer is disclosed to include a bottom frame formed of two side bars and a front bar and a rear bar, two vertical side frames each having two bottom frame bars respectively pivoted to the side bars and an arched top frame bar pivoted to the bottom frame bars, a transverse top bar detachably connected to the arched top frame bars of the two vertical side frames, a trailer bar pivoted to the front side of said side bar of the bottom frame and selectively locked between an extended position and a received position, and a bumper detachably connected between the side bars of the bottom frame.