Surface roughness of coldrolled steel strip is an important surface quality parameter, so the online measurement of surface roughness has become the key technology for high quality car plate and strip production. The traditional measurement of strip surface roughness uses offline test method, which will do harm to the steel surface and slow down the response speed. To solve the problem, novel methods based on optical mechanism, such as light cutting method, optical interferometry, speckle method and scattering method, are proposed for the online realtime measurement. In recent years, the strip surface roughness online measurement has made considerable progress in theory, model building, experiment and analysis of calibrations, but the contradiction between the roll of temper rolling operation control (such as roller shape, running speed) and the precision of online detection is still a difficult task for real time measurement and control. In view of the actual demand of surface roughness online measurement, the research results in theory are analyzed and the characteristics and adaptability of the corresponding measurement equipment are compared in this paper. And we put forward the feasible schemes to effectively solve the problem of realtime measurement and give an outlook of the theory of strip surface roughness online measurement and its engineering practice development trend.