The diffraction patterns of two systems——NaCl-H2O and NaCl-KCl-H2O at low temperature were determined with in situ XRD. The details of systems’ phase transition were further obtained. The solids were analyzed according to the patterns. For NaCl-H2O system, the phase transitions were different because of their different compositions. For the solution of 26.25% NaCl the solids included ice, NaCl, NaCl·XH2O(X<2), NaCl·2H2O in temperature range of -20~-25 ℃, for the solution of 25.70% NaCl the phase transition did not occur under experimental conditions. For NaCl-KCl-H2O system, the phase transitions were different too. For the solution of 20.03% NaCl and 10.19% KCl the solids were KCl and NaCl in temperature range of 0~-25 ℃, for the solution of 22.40% NaCl and 7.28% KCl the solid was KCl only in temperature range of -10~-25 ℃, as for the solution of 21.90% NaCl and 6.46% KCl there was no phase transition under the experimental conditions. The results show that there are some differences between the experimental and theoretical results due to their different states. The combination of sodium chloride with water results in the appearance of NaCl·2H2O. Crystallization processes as well as temperature control the occurrence of the phase transition. The phase transitions of the systems are results from comprehensive influences of several factors.