The theoretical analysis and experimental result of self-mixing laser speckle interference (SMSI) velocimeter for flow measurement are presented based on the theory of Fabry-Pérot (F-P) cavity model. The mean speckle frequency reflects the activity of flow. The variation of gain and distribution of probability density are given. The computer simulation shows the linear relation between the mean speckle frequency and velocity of flow, and experimental result proves relation. The velocity of glutin solution is measured by this linear relation. The effect of background and concentration of flow are considered in the experiment. When the concentration of flow is appropriate, the error of experiment is less than 8%. The results of experiment show this method is useful for measurement of flow.