Speckle patterns will arise on the screen of the laser projection display while a laser beam is scattered by the elements, which is once scattering speckle. Twice scattering speckle patterns will arise on the retinas of the audience’s eyes after the laser beam is scattered by a screen again. Both the speckle patterns could be suppressed by a random phase plate modulating the laser beam in illuminator of the laser projection display. A simplified model was built. It was deduced in theory that the contrast of twice scattering speckle patterns decreased while the random phase plate was rotating. It shows that the contrasts of once scattering speckle patterns and twice scattering speckle patterns obey the same rule for a certain system. Simulations were carried out, which confirmed the result. The contrasts of once scattering speckle patterns are 0.009 6 and the contrasts of twice scattering patterns are 0.019. It shows that once scattering speckle patterns and twice scattering speckle patterns could be suppressed while the laser beam is modulated with a random phase plate. A confirmation experiment was performed.