The invention relates to a butterfly valve pit with a lengthening bar. A butterfly valve is arranged in the butterfly valve pit. The butterfly valve pit comprises a pit chamber cover plate, the lengthening bar, a shaft, a concrete cover plate, a protection cover and a backfill soil layer. The shaft is arranged between the pit chamber cover plate and the concrete cover plate. The lengthening bar vertically penetrates the concrete cover plate, the shaft and the pit chamber cover plate from top to bottom, and the bottom of the lengthening bar is connected to the butterfly valve. A split collar is arranged on the upper portion of the lengthening bar and fixed to the concrete cover plate through expansion bolts. The top of the lengthening bar is covered with the protection cover, and the protection cover is fixed to the concrete cover plate. The backfill soil layer is arranged outside the protection cover. The butterfly valve pit with the lengthening bar is simple in structure, and subsequent maintenance is facilitated so that operators can control the butterfly valve to be opened and closed on the ground.