Disodium hydrogen phosphate dodecahydrate (DHPD) is a kind of phase-change hydrated material that has been widely used in heat-storage technology, but it has the common problems of supercooling and phase-separation of hydrated salts; therefore, the addition of a nucleating and thickening agent is a traditional method to solve the aforementioned problems. In this article, sodium carboxymethylcellulose (CMC) and xanthan gum (XG) are used to improve the supercooling and phase-separation properties of the hydrated phase-change salts. The phase transition characteristics and cycling stability are analyzed in detail with the solidification curve, differential scanning calorimetry (DSC), X-ray diffraction (XRD), thermal gravimetry (TG), and thermal cycling experiments. The thickening water absorption of CMC and XG decreases the supercooling properties of DHPD without the addition of nucleating agents. The addition of 5% XG together with 2% CMC reduces the supercooling of DHPD to 1.6 °C. DSC analysis showed that the additions can adsorb the free water, decrease the evaporation of crystalline water, and remove the self-phase separation problems. The phase-change temperature and latent heat were 36.2 °C and 201.5 J/g, respectively. The supercooling degree of the modified DHPD was no more than 2 °C, showing its excellent thermal stability in the accelerated thermal cycle experiments.