The disclosure provides a collimator assembly, comprising at least at least two collimators configured to be moveable relative to each other such that the collimator assembly is switchable between at least two collimation modes; in respective collimation modes, the at least two collimators are superposed with each other in a thickness direction of the collimator assembly, such that the collimator assembly presents different combined patterns for collimating and shielding rays incident onto the collimator assembly and that the collimator assembly has corresponding ray shielding thickness for effectively shielding rays.