A reclining chair with a backrest tilting device that comprises a seat, a leg and a backrest, said backrest tilting device comprising: a support supported by the leg and supporting the seat of the chair; a pivot pivotally secured to the support and an arm extending substantially perpendicular to said pivot; and a pair of backrest support rods in which a front lower end is fixed to the pivot, a rear end of the pair of backrest support rods being Z coupled to the backrest, a force-promoting unit of the support forcing the pair of backrest support rods in a direction where the backrest is upright, characterized in that: the force-promoting unit comprises a rotation-promoting unit and a gas spring, wherein the rotation -promoting unit comprises a rubber torsion spring comprising the pivot as a core, an outer tube fixed to the support and elastic material filled between the outer tube and the pivot, said pivot being coaxial with and turning with respect to the outer tube to allow the elastic material to be elastically deformed to apply return-turning force to the pivot; and the gas spring has a front end, a rear end and a knob at the front end, the front end being coupled to the support, the rear end being coupled to an end of the arm of the pivot, the knob moving to an active position to enable the gas spring to extend and contract and returning to a passive position not to enable the gas spring to extend and contract. a support supported by the leg and supporting the seat of the chair; a pivot pivotally secured to the support and an arm extending substantially perpendicular to said pivot; and a pair of backrest support rods in which a front lower end is fixed to the pivot, a rear end of the pair of backrest support rods being Z coupled to the backrest, a force-promoting unit of the support forcing the pair of backrest support rods in a direction where the backrest is upright, characterized in that: the force-promoting unit comprises a rotation-promoting unit and a gas spring, wherein the rotation -promoting unit comprises a rubber torsion spring comprising the pivot as a core, an outer tube fixed to the support and elastic material filled between the outer tube and the pivot, said pivot being coaxial with and turning with respect to the outer tube to allow the elastic material to be elastically deformed to apply return-turning force to the pivot; and the gas spring has a front end, a rear end and a knob at the front end, the front end being coupled to the support, the rear end being coupled to an end of the arm of the pivot, the knob moving to an active position to enable the gas spring to extend and contract and returning to a passive position not to enable the gas spring to extend and contract. the force-promoting unit comprises a rotation-promoting unit and a gas spring, wherein the rotation -promoting unit comprises a rubber torsion spring comprising the pivot as a core, an outer tube fixed to the support and elastic material filled between the outer tube and the pivot, said pivot being coaxial with and turning with respect to the outer tube to allow the elastic material to be elastically deformed to apply return-turning force to the pivot; and the gas spring has a front end, a rear end and a knob at the front end, the front end being coupled to the support, the rear end being coupled to an end of the arm of the pivot, the knob moving to an active position to enable the gas spring to extend and contract and returning to a passive position not to enable the gas spring to extend and contract.