Due to the uncoordinated deformation of aero-engine casing and rotor, it is imperative to measure the tip-clearance and control it dynamically. In this paper, the current research status of tip-clearance at home and abroad were introduced, including the common tip-clearance measurement methods, their measuring principles, application conditions, advantages and disadvantages. In mentioned measurement methods, the one based on optical measurement technology has the advantages in high precision, strong adaptability, corrosion resistance, high temperature resistant, and immunity to the electromagnetic interference. Meanwhile non-contact measurement can be realized by it. The optical measurement has become the research focus in the field of tip-clearance measurement technology, and should be vigorously developed.