In this work, we demonstrate a novel design to investigate interfacial reaction of optical activity materials, utilizing a Sagnac interferometer assisted with surface plasmon resonance based sensor technique. Upon application to interrogate optical activity, the type-2 optical configuration of close loop in this work can entirely encircle the interaction zone of surface plasmon resonance reacting with optical activity medium, while the type-1 optical configuration of close loop does partially. The greater the geometrical phase owing to the encircled interaction zone between optical activity medium and polarized light, the larger the phase shift of interference pattern modulated by the concentration of optical activity medium. The slope of phase shift in interference phase pattern of p-wave versus concentration of dextrose solution determined with this method is greatly improved, 3000 times better than that with Lee's method, 18600 times with Lin's method, and 222000 times with Chou's method. Besides, the pinch point of phase shift vs. concentration indicates the ending of interference pattern phase shift as the concentration of each tested optical activity medium keeps climbing up, and can reflects the strong dependence of molecular shape during interfacial adsorption.