The lithium–sulfur batteries show the great potential to be the most promising candidate for high energy applications. However, the shuttling of soluble polysulfides deteriorates the battery performance tremendously. To suppress the diffusion of soluble polysulfides, diatomite that has abundant natural three-dimensional ordered pores is incorporated into the cathode to trap polysulfides. The composite cathode material (S-DM-AB for short), including sulfur (S), diatomite (DM), and acetylene black (AB) is prepared by an impregnation method. For comparison, another composite cathode material (S-AB for short) including sulfur and acetylene black is also prepared by the same method. The battery with S-DM-AB composite cathode material delivers a discharge capacity of 531.4 mAh/g after 300 cycles at 2 C with a capacity retention of 51.6% at room temperature. By contrast, the battery with S-AB composite cathode material delivered a capacity of only 196.9 mAh/g with a much lower capacity retention of 18.6% under the same condition. The addition of diatomite in the cathode is proved to be a cheap and effective way to improve the life time of the lithium sulfur batteries.