The utility model discloses a but main shaft of reducing honing cutter, it includes: the main shaft cover, it is equipped with preceding interface, well interface and back interface, the main shaft body, it includes: the footstalk, and the spindle nose, in the main shaft cover was located to its cover, it was equipped with corase grind hydraulic pressure chamber and correct grinding hydraulic pressure chamber, hydraulic pressure hole and rear portion were equipped with back hydraulic pressure hole during hydraulic pressure hole before the front portion in corase grind hydraulic pressure chamber is equipped with, the front portion in correct grinding hydraulic pressure chamber were equipped with, it advances oil ring groove respectively to be equipped with one in the drill way in three hydraulic pressure hole department on the lateral wall of spindle nose, the corase grind push rod, corase grind hydraulic pressure intracavity is located through a corase grind piston headgear in its rear end, the corase grind push rod is equipped with the correct grinding guiding hole, and the correct grinding push rod, its rear end locates correct grinding hydraulic pressure intracavity through a correct grinding piston headgear and the correct grinding guiding hole is passed in its activity. The utility model discloses a combination formula hydraulic control promotes corase grind push rod and correct grinding push rod to carrying out the reducing to honing head and adjusting, its transmission is reliable and stable, and so as to guarantees the machining precision.