We propose a system to eliminate the graininess of an integral imaging 3D display by using a transmissive mirror device (TMD). The proposed system consists of a 2D display, a micro-lens array (MLA), and a TMD. The TMD comprises square apertures with mirror-reflective inner wall. The light rays pass through the square aperture to form a diffraction spot, and the diffraction light intensity has a Sinc-function distribution. Therefore, the TMD can be used as an optical low-pass filter. In a certain imaging range, the mainlobe of the Sinc-function distribution is almost unchanged. The TMD has the property of a volumetric optical low-pass filter. It can interpolate the interval between discrete 3D pixels. Therefore, the TMD can be used to eliminate the graininess. The resolution of the 3D image is improved by 2.12 times. The experimental results verify the feasibility of the proposed system.