Accurate measurement of fluid-velocity profile and flow rate in complex duct is important to master the velocity distribution law and its application. A scheme of measuring the fluid-velocity profile and flow rate in rectangular duct based on laser-diode self-mixing effect was proposed. Based on the self-mixing interference theory and velocity distribution of laminar flow in rectangular duct theory, the feature of frequency spectrum of self-mixing signal of laminar flow in rectangular duct was analyzed. The feature of fluid-velocity profile and flow rate in rectangular duct were obtained. The average relative error of measured fluid-velocity is 2.7%. The relative errors of measured flow rate in rectangular duct are less than 2.0%. Experimental measurements accord with the theory.