According to the compositions of the underground gasfield brines in the west of Sichuan Basin, the phase equilibria in the ternary systems KBr–K2B4O7–H2O and KCl–K2B4O7–H2O at 373 K were studied using the isothermal dissolution equilibrium method. The solubilities of salts and the densities of saturated solutions in these ternary systems were determined. Using the experimental data, phase diagrams and density-composition diagrams were constructed. The two phase diagrams were simple co-saturation type, each having an invariant point, two univariant curves and two crystallization regions. The equilibrium solid phases in the ternary system KBr–K2B4O7–H2O are potassium bromide (KBr) and potassium tetraborate tetrahydrate (K2B4O7·4H2O), and those in the ternary system KCl–K2B4O7–H2O are potassium chloride (KCl) and potassium tetraborate tetrahydrate (K2B4O7·4H2O). Comparisons of the phase diagrams of the two systems at different temperatures show that there is no change in the crystallization phases, but there are changes in the size of the crystallization regions. As temperature increases, the solubility of K2B4O7·4H2O increases rapidly, so the crystallization field of K2B4O7·4H2O becomes smaller.