Abstract: Sichuan Jiajika, Lijiagou and other places have rich spodumene reserves, it provides an important source of raw materials for downstream lithium battery energy storage materials and other fields.In the traditional spodumene production process of lithium carbonate, sodium hydroxide neutralization method is used to remove impurities such as iron and aluminum. However, the added value of by-product sodium sulfate is low, resulting in high production costs. This paper studies the process of replacing strong sodium oxide with ammonia gas to neutralize impurities to prepare lithium carbonate, it was found that LiNH₄SO₄ double salt was produced during the precipitation and crystallization process of refined lithium solution, which affected the yield of lithium and the quality of lithium carbonate products.In order to solve the problem, the stable phase equilibrium relationship of the ternary system Li⁺, NH₄⁺//SO₄^2--H₂O at 298 K was studied by the isothermal dissolved equilibrium method. The solubility, density and refractive index data of each component of the equilibrium liquid phase were measured, and the phase diagram, density-composition diagram and refractive index composition diagram of the system were drawn. The results show that this system is a complex ternary system, with the formation of LiNH₄SO₄ double salt; its stable phase diagram consists of 3 solid phase crystallization regions, 3 univariate curves, 2 invariant points, 3 crystallization regions corresponding to (NH₄)₂SO₄, Li₂SO₄·H₂O and LiNH₄SO₄, And double salt LiNH₄SO₄ crystallization area is the largest.The research results show that in order to avoid the production of LiNH₄SO₄ double salt, It is necessary to reduce the lithium-sulfur ratio (w(Li₂SO₄)·H₂O/w(NH₄)₂SO₄) in the lithium solution to less than 1 through the calcium ion precipitation method at the front end of the spodumene-made industrial-grade lithium carbonate process, the research results provide theoretical guidance for the use of ammonia neutralization and impurity removal in the Sichuan spodumene hydrometallurgical process.