Cu₂CoSnS₄ (CCTS) and Cu₂NiSnS₄ (CNTS) can be regarded as Cu₂ZnSnS₄ compounds with the Zn²⁺ replaced by Co²⁺ and Ni²⁺, and are alternative absorber materials in thin film solar cells. Here, we fabricated high-quality CCTS and CNTS nanocrystal thin films by employing the ion substitution strategy. We systematically studied the crystal structures, surface morphologies and compositions of the as-prepared and selenized thin films via X-ray diffractometry (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS). The selenized CCTS and CNTS films showed an obvious phase separation process for the oxidizing reaction of Co²⁺ and Ni²⁺ with high-temperature selenium steam, and then formed Cu₂Sn(S,Se)₃, Co(S,Se)₂ and Ni(S,Se)₂ compounds. We successfully fabricated photodetector devices using CCTS and CNTS nanocrystal films, and achieved a strong and reversible photoresponse performance.