A robust optical insertion loss measuring approach based on a symmetrically coupled add-drop microring resonator is demonstrated on silicon-on-insulator platform. Utilizing resonant wavelengths and relative values of measured optical power, this approach frees the insertion loss measurement from the uncertainties caused by experimental set-up, including system alignment and wavelength dependence of the couplers. Strip-slot mode converters were fabricated and measured to present the exemplary insertion loss measurement process. A series of experimental results confirm that the insertion loss of the low-loss silicon photonic devices can be accurately and reliably obtained even the adopted couplers are wavelength-dependent, and the fibers are deviated 15 μm from the horizontal direction and 110 μm from the vertical direction, exhibiting excellent robustness to the experimental set-up.