The disclosure provides a mask assembly comprising a mask which comprises a first surface facing towards an evaporation source and a second surface facing towards an article to be deposited, and pattern openings are formed in the mask, wherein the mask assembly further comprises at least one test sheet which is detachably provided on the mask. The test sheet is provided at a region on the mask where no pattern opening is provided, and is configured such that an evaporation material can be deposited on the test sheet when an evaporation process is performed by using the mask assembly. The disclosure further provides an apparatus and a method for detecting a film thickness of the evaporation material on the mask assembly. With the apparatus, the film thickness of the evaporation material deposited on the mask assembly can be easily detected.