The invention discloses an opening size adjustable spanner and relates to the spanner. The opening size adjustable spanner comprises a handle portion, wherein one end of the handle portion is provided with a head portion; the head portion comprises a ring; six first through holes are uniformly formed in the inner side wall of the ring in the periphery direction; each first through hole is internally provided with a cylinder; one end of each cylinder is provided with a baffle board; second through holes are symmetrically formed in the two ends of each baffle board; a steel rope penetrates through the second through holes in the six baffle boards. Compared with the prior art, the opening size of the opening size adjustable spanner can be adjusted according to the actual requirements, the requirements of production and machining are met, the structure is reasonable, the operation is convenient, the working efficiency is greatly improved, and the industrial production is achieved.