This paper introduces one on-line measuring system of the metal surface roughness. The system is based on the theory about light's diffusing of the metal surface. Laser is used as the lamp-house of measure and in the system detector collects the light's signals by optical fiber. The surface roughness measuring principle and method are presented and the experiment data are analyzed. After comparing with other approaches, the standard-sample-plate comparison is adopted. The method is simple in structure and high in efficiency and the one-by-one measuring points can reflect the real surface. As for all the factors can induce errors in on-line measuring, the research obtains good result by revising the errors with reparative formula.