The invention discloses a double ratchet wheel type telescopic obstacle-crossing support arm and an obstacle-crossing robot. The double ratchet wheel type telescopic obstacle-crossing support arm comprises a telescopic support arm and a support arm body which are in clearance fit. A rear travelling wheel is fixed on the support arm body and is axially connected with a power output shaft in series. A front travelling wheel is fixed on the telescopic support arm and is axially connected with a transmission shaft in series. An obstacle-crossing crawler belt is sleeved with the front travelling wheel and the rear travelling wheel, a tension roller mechanism is fixed on the support arm body, a double-ratchet wheel mechanism is fixed at one end of the support arm body close to the telescopic support arm; the double-ratchet wheel mechanism drives a rack to move left and right under control of a motor encoder, one end of the rack is connected with the telescopic support arm and the other end of the rack is connected with a cam. The cam is matched with the tension roller mechanism tangentially, the rack moves left to right to drive the cam to move left to right and the telescopic support arm to slide telescopically; the cam moves to drive the tension roller mechanism to slide up and down along the cam, and finally the obstacle-crossing crawler belt relaxes and tights to be matched with telescopic sliding of the telescopic support arm.