The invention provides a collimator, comprising a fixed block and a movable block, wherein a variable space used for containing an additionally arranged part is formed between the fixed block and the movable block; and the size of the variable space is flexibly changed along with the position change of the movable block, so that the variable space can adapt to different specifications and types of additionally arranged parts. Compared with the prior art, the collimator adopts a structural design that the fixed block and the movable block are arranged in a separated way, so that the variable space is available in the collimator, and the collimator can adapt to the additionally arranged part with a larger axial dimension; and furthermore, the variable space can be flexibly adjusted according to the size of the additionally arranged part, so that the internal space of the collimator is better saved, and reasonable arrangement is facilitated to be realized in the collimator.