A flexible support device for chair back tilting, comprising: a pair of driving members , for connecting to two opposite sides of the chair back (A2); a base , having a pair of partition plates separately corresponding to the driving members , wherein each of the driving members is pivotally connected to one of the partition plates with a first pivot ; a spring disposed in the base ; and a pulling rod , crossing the base , two ends of the pulling rod separately connected to two free ends of the two driving members , having a frame encompassing the spring , and the frame having an action end blocking the spring , wherein the action end is formed with a through hole for receiving a shaft to assemble the frame into the base , and the spring is put around the shaft ; thereby when the chair back (A2) is pressed to tilt rearward, the driving members are rotated to draw the pulling rod to move toward the chair back (A2), and the action end of the frame is driven to compress the spring so as to make the chair back (A2) flexibly supported; wherein each of the partition plates is formed with a guiding hole which is located away from the driving members , each of the guiding holes is passed by a second pivot to connect with a bottom of the cushion (A3), and the guiding holes allow the second pivots to move therein, and each of two ends of the driving members is pivotally connected to the bottom of the cushion (A3) with a third pivot to make the driving members rotatable relative to the cushion (A3); characterized in that an end of the shaft is formed with a thread for threadedly connecting a pressing disk which blocks an end of the spring . a pair of driving members , for connecting to two opposite sides of the chair back (A2); a base , having a pair of partition plates separately corresponding to the driving members , wherein each of the driving members is pivotally connected to one of the partition plates with a first pivot ; a spring disposed in the base ; and a pulling rod , crossing the base , two ends of the pulling rod separately connected to two free ends of the two driving members , having a frame encompassing the spring , and the frame having an action end blocking the spring , wherein the action end is formed with a through hole for receiving a shaft to assemble the frame into the base , and the spring is put around the shaft ; thereby when the chair back (A2) is pressed to tilt rearward, the driving members are rotated to draw the pulling rod to move toward the chair back (A2), and the action end of the frame is driven to compress the spring so as to make the chair back (A2) flexibly supported; wherein each of the partition plates is formed with a guiding hole which is located away from the driving members , each of the guiding holes is passed by a second pivot to connect with a bottom of the cushion (A3), and the guiding holes allow the second pivots to move therein, and each of two ends of the driving members is pivotally connected to the bottom of the cushion (A3) with a third pivot to make the driving members rotatable relative to the cushion (A3); characterized in that an end of the shaft is formed with a thread for threadedly connecting a pressing disk which blocks an end of the spring . <EOS>