Co²⁺:ZnS sulfide nanocrystals are prepared by a simplified hydrothermal method without organic materials. These nanocrystals have a cubic sphalerite structure with an average grain size of around 8.3 nm. Mid-infrared fluorescence emissions at 3 400 nm and 4 700 nm were detected under 808 nm laser pumping. By heat treatment at 800℃ in a reducing atmosphere, the prepared nanocrystals transfered to cubic sphalerite and wurtzite mixed crystal forms, and the average grain size increaes to 22.5 nm. After the heat treatment, the nanocrystals possess lower hydroxyl content on surface, therefore exhibit stronger mid-infrared fluorescence. This method is facile and aviods the introduction of organic mid-infrared fluorescence quenching centers during the preparation of Co²⁺:ZnS nanocrystals. The surface defects and hydroxy content of Co²⁺:ZnS nanocrystals were reduced and more stronger mid-infrared fluorescence was obtained after heat treatment.