Abstract: The irradiation field adjustable collimator was designed based on the 6 MeV crack detection accelerator in this paper. The collimator is mainly composed of four collimation blocks. Every collimation block has 2 degrees of freedom (DOF). The collimation block shields stray X-ray with tungsten-nickel iron of 43 mm thickness. The shape and size of the X-ray irradiation field are automatically adjusted by controlling the vertical and horizontal motors. And the irradiation field adjustable collimator can meet the need of X-ray sources for different fields in industrial inspection and industrial CT. The collimation blocks were arranged in the same plane and staggered from end to end. This structure of collimator can effectively reduce the leakage of X-ray.