Disclosed are an apparatus and a method for automatically adjusting the tension on a mining elevator flexible guide rail. The apparatus comprises an automatic tightening apparatus (1) at the opening of the well, and a fixing apparatus (2) on the bottom of the well, respectively connected to two ends of the steel cable (3). The automatic tightening apparatus (1) at the opening of the well is connected to a hydraulic system (5) and to a tension measuring system (4). The automatic tightening apparatus (1) at the opening of the well comprises a lower retainer seat (1-14) arranged on an upper bearer beam (1-1), a tightening oil cylinder body (1-2) and a plunger thereof (1-3), a pin shaft retainer (1-11), a support frame (1-6), a fall-proof holder (1-13), a pin shaft holder (1-5), a fall-proof cable retainer (1-12), a tightening fixed cable retainer (1-10), two guide columns (1-7) and guide grooves thereof, an upper retainer seat (1-9), and a cable orientation-adjusting cable retainer (1-8). The fixing apparatus (2) on the bottom of the well comprises a fixing plate (2-3), and a fixed cable retainer (2-5). The apparatus and the method allow the flexible guide rail to be structurally simple, convenient to process, reliable in performance, and easy to install and to maintain. The apparatus and the method allow restriction of vertical orientation and horizontal deflection, and are capable of real-time detection and automatic adjustment of the tension in the flexible guide rail of the elevator.