To maintain the defocusing amount during the metal laser cladding forming experiments, the distance between the forming surface and the nozzle should be fixed. Based on optical triangle method, an embedded machine vision online measuring system for controlling the laser defocusing amount is proposed. The formula for designing the lens focal length of the measuring camera is deducted, and both hardware and software solutions are meanwhile presented. To process the gained pictures from the measuring camera effectively and stably, a new algorithm estimation based on molten pool gray threshold is used for programming. The output voltage from the measuring system is also integrated into the robot controlling coding which helps adjust the nozzle position automatically once the laser defocusing amount is offset. This measuring system is later verified through several metal laser cladding forming experiments to be stable and precise. By comparing the thin-wall cylinder forming parts, the surface roughness of the part made with the measuring system is much brighter and cleaner than the ones without this system.