Speckle, severely affecting the image quality in laser projection, would arise when coherent lights are scattered from a rough surface. Vibrating or rotating a diffuser or a diffractive optical element, the conventional method of speckle suppression (hereinafter refered to as “the conventional method”), does reduce the speckle, however, also causes energy loss. The incompatible relationship between speckle suppression and energy preservation, reflected by using the conventional method, is analyzed theoretically and practically. In order to achieve both speckle suppression and energy preservation, a novel method, rotating a light pipe, which is in principle by averaging independent speckle patterns (hereinafter refered to as “the novel method”), is proposed and demonstrated from theoretical and experimental aspects. Experimental results indicate that the novel method degenerates coherence of the source, keeps polarization of the source, brings the speckle contrast down to 3.45%, and preserves energy.