Abstract: An adjustable flexible back cushion including a flexible protecting frame, a ventilating mesh, an upper pull belt, a lower pull belt, fasteners and elastic belt, wherein the protecting frame contains a flexible metal strip and the left and right sides thereof are curved while the lower portion thereof is bent rearward, and at least two pull belt assemblies each of which is composed of the upper and lower pull belts and the fasteners are dispowed on the protecting frame in vertical direction while the elastic belt is sewn on the protecting frame across over the pull belt assemblies, whereby the back cushion can be adjusted to eliminate a user's backache caused by improper sitting attitude, the back cushion being suitable for a car seat, office chair and general domestic chair.