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As a promising material, Cs4PbX6(X = Cl, Br, I) nanocrystals (NCs) have attracted much attention. However, their luminescent property is still under debate. In this work, we first systematically studied the colloidal preparation of Cs4PbX6NCs. It is found that the critical parameter for the formation of Cs4PbX6NCs is the ratio between Cs and Pb. Pure Cs4PbX6NCs are nonluminescent. The luminescence property of previous reported Cs4PbX6NCs may come from the impurity of luminescent CsPbX3NCs. No coexistence of both Cs4PbX6and CsPbX3phase has been found in one single nanoparticle. The water-triggered transformation from nonluminescent Cs4PbX6NCs to luminescent CsPbX3NCs has been quantitatively studied. The potential application of Cs4PbX6NCs in humidity sensor and anticounterfeiting have been demonstrated. This work is important because it not only confirmed the nonluminescent nature of Cs4PbX6NCs but also demonstrated the potential application of such NCs.