Measurement of IR optical spot is of great importance to physics and technology. In this paper, we present a reticle method for exactly measuring Gaussian optical spot parameter. Reticle method may be used for fast, accurate measurement of Gaussian optical spot parameter. Suppose the brightness of a spot is a Gaussian model, when optical spot scans over the bars reticle, the parameter of Gaussian optical spot can be exactly known with measuring peak value of frequency spectrum.