The utility model relates to a simple reducing flashboard for well drilling, particularly the corollary equipment of oilfield drilling engineering. The utility model is composed of a front sealing and a top sealing, wherein, the top sealing is coupled with the front sealing through a bolt. The front sealing is composed of a left sizing block, a right sizing block and inner sizing blocks which are vulcanized in the rubber of the front sealing. The left sizing block and the right sizing block are respectively positioned at both sides of the front sealing, and space exists between the inner sizing blocks. The utility model has the advantages that a metal sizing block is arranged in rubber, and space in which the rubber can be deformed is arranged in advance in the rubber, therefore, the sizes of the sealed diameter can be changed within certain limit; the ranges of variable diameter are larger, and the pressure to be born is larger; the utility model is reliably sealed. Using the flashboard of a spray preventing device can reduce the number of the flashboards which are arranged and the operational process of changing the flashboards. The utility model has the advantages of increased work efficiency, simple structure and convenient manufacture and is used in well drilling work.