According to the composition of the brine resources in the west of Sichuan Basin, phase equilibria in the quaternary system Na^{+},K^{+}//Br^{-},SO_{4}^{2-}-H_{2}O at 373 K is measured by the isothermal solution saturation method, and the solubilities and densities of the solution are determined experimentally. Using the experimental results, the dry salt phase diagram, water diagram and the densities versus composition diagram are obtained. In the phase diagram of quaternary system Na^{+},K^{+}//Br^{-},SO_{4}^{2-}-H_{2}O at 373 K, the double salt Na_{2}SO_{4}·3K_{2}SO_{4} is found. There are three invariant points, seven uninvariant curves and five crystallization fields in the quaternary system. The five crystallization fields correspond to NaBr, Na_{2}SO_{4}, K_{2}SO_{4}, KBr and Na_{2}SO_{4}·3K_{2}SO_{4}(Gla), respectively. The crystallization field of NaBr has the smallest crystallization area, whereas the double salt Na_{2}SO_{4}·3K_{2}SO_{4}(Gla) has the biggest crystallization field in the quaternary system. It means that the double salt Na_{2}SO_{4}·3K_{2}SO_{4}(Gla) has the smaller solubility, and it can be easiest separated from solution. Compared with the two phase diagrams of quaternary system Na^{+},K^{+}//Br^{-},SO_{4}^{2-}-H_{2}O at 323 K and 373 K, the result shows that the numbers of invariant points, crystallization fields and unvariant curves are the same. The double salt all forms in the phase diagrams of quaternary systems at two different temperatures. But univariant curve GH of the corresponding balance solid phase is different and the crystal water of sodium bromide has disappeared at 373 K. In comparison with the quaternary system Na^{+},K^{+}//Cl^{-},SO_{4}^{2-}-H_{2}O and the quaternary system Na^{+},K^{+}//Br^{-},SO_{4}^{2-}-H_{2}O at 373 K, the two phase diagrams have very similar shapes, each of them having three invariant points, seven univariant curves and five crystallization fields. The crystallization field of the salt NaBr is apparently smaller than that of NaCl. It is also found that halide has the salting-out effect on sulfates. The water content and the density transformation rules are discussed simply. The water content is lower with the higher solution of bromine and the density is higher with the higher solution of bromine.