Abstract: The whiteness of diatomite has important influence on its application. The alkali dissolving process was used to improve the whiteness of Yunnan diatomite, and the effect of the time of alkali-dissolved reaction, the temperature of alkali-dissolved reaction, the liquid-solid ratio of alkali-dissolved reaction and the alkali-soil ratio of alkali-dissolved reaction on the whiteness of Yunnan diatomite was studied. The structure characterization of the samples were obtained by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM) and low temperature nitrogen adsorption. According to the results of characterization, the whitening mechanism of alkali dissolving process was analyzed. The experimental results show that the optimum condition are:reaction time 30 min, reaction temperature 80℃, alkali-soil ratio 0.28 GA6FA 1, liquid-solid ratio 5 GA6FA 1. The whiteness of diatomite is proved from 62.7% to 71.3%, and the specific surface area and pore structure characteristics of diatomite are obviously improved on the premise of the preservation of the pore structure. It could be concluded that the whitening mechanism is that the surface of diatomite particles is coated with a layer of amorphous nano-silica particles.