A chair supporting structure comprises a base frame having a plurality of connecting arms which is downwardly inclined; a lower distal end of each connecting arm being formed with outer threads for combining with the connecting units; a plurality of legs being installed at a bottom of the a seat of a chair; an upper end of each leg being formed with an insertion hole; and a plurality of combining units for combining the base frame and let set; an upper end of the combining unit being formed with a combining hole which has inner threads; so that the outer threads of each combining arm being threadly engaged with the inner threads of the combining hole of the combining unit; a lower end of the combining unit being tightly engaged to the insertion hole so that the base frame is combined with the legs.