The utility model discloses an ejecting device with the adjustable length. The device comprises a plurality of pipe bodies connected end to end, a sliding groove in the axis direction of the pipe bodyis formed in the upper portion of the pipe body. A plurality of fixing holes distributed in the axis direction of the pipe body are formed in the lower portion of the pipe body. The lower portion ofeach pipe body is inserted into the top of the pipe body connected with the lower portion of the pipe body and connected with the sliding groove by inserting connecting pieces into the fixing holes, the two ends of each connecting piece extend out of the sliding groove, the up-down sliding area of each connecting piece is limited through the sliding groove, and limiting sleeves for limiting downward movement of the connecting pieces are arranged outside the sliding groove in the upper portion of the pipe body. The utility model relates to a length-adjustable jacking device. The formwork systemand the inner frame system are prevented from being influenced by load concentration in construction, potential safety hazards in the concrete pouring process are eliminated, the needed position canbe directly reinforced from the ground during installation, and the formwork system and the inner frame system are simple, practical, suitable for various conditions and convenient to operate, and canbe adjusted according to the height needed by back-jacking supporting.