A new method with a one wavelength laser for making 3D diffusion objects true color rainbow hologram on the photoresist plate is presented. In this method, we have utilized the synthesized slit technology to record the true color rainbow hologram. It results in higher energy utilization and good color reproduction of the reconstructing image. So, it is valuable in fabricting 3D diffusion objects true color embossed rainbow hologram. A satisfactory experimental result is obtained and the analysis is given.