Dicalcium phosphate dihydrate (CaHPO₄·2H₂O, DCPD) is an important inorganic material widely used in the food, feed, pharmaceutical, and agricultural industries. The CaCO3-H3PO4 neutralization method is commonly employed for its industrial production. However, the products often exhibit poor thermal stability. This study investigates the effects of reaction time and temperature on the purity of DCPD. Impurity contents in the samples were quantitatively analyzed by thermogravimetry (TG), and the initial thermal decomposition activation energies were determined using both the Kissinger and Flynn-Wall-Ozawa (FWO) methods. Furthermore, the thermal stability of products with varying purity levels was compared. Quantitative TG analysis shows that high-purity DCPD (99.5 wt.%) can be obtained at 40℃ with a reaction time of 30 minutes. Insufficient reaction time (less than 30 min) leads to residual CaCO₃, whereas an excessively high reaction temperature (above 40℃) promotes the formation of anhydrous dicalcium phosphate (CaHPO₄, DCPA) along with residual CaCO₃. Calculation of the initial thermal decomposition activation energies reveals that the presence of impurities lowers this activation energy, thereby reducing the thermal stability of the product. Thermal stability tests reveal that impurities formed during the neutralization process significantly reduce the thermal stability of DCPD. Compared with the high-purity sample, impurity-containing samples show markedly higher water loss rates after isothermal treatment at 120℃ for 4 h and under simulated transportation conditions (60℃ for 12 h followed by cooling to room temperature for 12 h, repeated for three cycles). This work provides an experimental basis for producing high-quality DCPD via the CaCO3-H3PO4 neutralization method.