Total-reflection X-ray optics play an important role in X-ray microscopy technology, and the study on their resolution limit is helpful for both designers and users. Theoretical study on the resolution limit of total-reflection X-ray optics is presented based on the Heisenberg uncertainty principle. The theoretical results show that the resolution limit of total-reflection X-ray optics depends on material. The focal spot size limits of total-reflection X-ray optics made of nickel, lead glass and borosilicate glass are 3.2, 4.2 and 6.6 nm, respectively.