An under balance down-hole casing valve for measuring bottom-hole pressure is provided. The inner surface of an upper outer cylinder (3) is processed into steps. A pipe valve assembly (5) is arranged in the annulus formed by the upper outer cylinder (3), a lower outer cylinder (4) and an inner cylinder (2). A boss is arranged in the chamber of an upper connector (1), and another boss is arranged on the outer surface of the inner cylinder (2). The upper outer cylinder (3) and the inner cylinder (2) are clearance-fitted. The boss in the inner chamber of the upper connector (1), the boss on the outer surface of the inner cylinder (2) and the inner surface of the upper outer cylinder (3) form a hydraulic chamber (8), the lower part of the boss of the inner cylinder (2) and the inner chamber of the upper outer cylinder (3) form another hydraulic chamber (9). Plural axial bores are provided on the upper end of the upper outer cylinder (3). The bores are connected with a hydraulic control system via a pipeline, wherein two bores are respectively communicated with the hydraulic chamber (8) and the hydraulic chamber (9). The bore (7) is a stepped hole, which is communicated with the annulus formed by the upper outer cylinder (3) and the pipe valve assembly (5). When the casing valve is turned off, the bottom-hole pressure can be monitored via the upper outer cylinder (3) and the bore (7).