This paper proposes a new method for no line measuring geometrical size of opaque cylindrical object (such as the cable).Using the He Ne laser as the optic source,using CCD as a detector,and using adopting double pass spectral method,the geometrical size of object is detected,the actual problems for how the parallel optic beam to cover full pixels of CCD are analyzed.In this paper,a splite composition and double pass spectral optic system are proposed.And the simple calculative formula is also given in this paper.The advantages of this measuring method are:high resolution and accuracy,free scaning,contactless and good stability etc.