A length adjustable connection structure contains: an adjustment assembly, an engagement assembly, and a receiving tube. The receiving tube includes an insertion accommodated, a coupling portion, a groove, a trench, and an elongated slot. The engagement assembly includes a fixing portion, a toothed portion, a fitting portion, a hexagonal hole, and a circular hole. The adjustment assembly includes a connector, a locating orifice, a hexagonal column, and a C-retainer. The hexagonal column of the adjustment assembly is received in the hexagonal hole of the engagement assembly so that the engagement assembly is connected on the hexagonal column, the C-retainer is engaged in the notch of the hexagonal column, and the adjustment assembly and the engagement assembly are accommodated in the receiving tube, hence the toothed portion of the fixing portion is engaged in the groove of the coupling portion, and the fitting portion is fixed in the trench.