The foldable bed frame contains two frame assemblies side-by-side and laterally joined together. Each frame assembly contains an upper assembly and a lower assembly. Upper beams from the two upper assemblies are end-to-end joined together through upper connection pieces. Lower beams from the two lower assemblies are end-to-end joined together through lower connection pieces and intermediate connection pieces between the lower beams and the lower connection pieces. The various connection pieces are light-weighted and reliable, allowing the bed frame to be folded and unfolded smoothly and conveniently.