Demodulation algorithm is a very important part in the Sagnac interferometer system. In the demodulation process of some digital open-loop algorithms, some special phase points will confuse with each other and cause an error output value. Based on the theoretical analysis of special phase points, a virtual optical-fiber Sagnac interferometer system with integrated circuit chip is designed and implemented. This system can easily solve the problem of special phase point measurement in demodulation system, and judge whether the demodulation system can generates an error in these special phase points. In the experiment, special phase point test is implemented to a digital-analog mixed demodulation system by the proposed. The scale factor of the virtual optical-fiber Sagnac interferometer is 49.9°/V. The nonlinear degree of the scale factor is 0.167%, and the asymmetry degree of the scale factor is 0.088062%. The zero drift of the system is 9.5265°/h.