Simultaneous phase-shifting interferometric microscopy is proposed herein to measure the phase of red blood cells for leveraging the characteristics of slow blood flow and a small number of red blood cells in microvessels. A characteristic volume model of red blood cells is established to match characteristic red blood cells from multi-frame phase maps. Then, slow blood flow detection is realized. A Ф100-μm microvessel prototype with a flow rate of 0.1-1.0 mm/s is prepared using Alsever’s solution of bovine red blood cell. A simultaneous phase-shifting interferometric microscopy experimental setup based on a micro-polarization array is constructed to validate the feasibility of the proposed detection method, and the measurement error of the blood flow velocity is no greater than ±11.2%.