In order to solve the problem of high chlorine content in filter cake during the production of basic magnesium carbonate from the old brine of Qarhan Salt Lake，a new idea of using the old brine of Qarhan Salt Lake to prepare basic magnesium carbonate was put forward.The differences of water consumption and chlorine content between one-step method and two-step method in the preparation of basic magnesium carbonate were studied，and the crystal shape and morphology of the samples were characterized by XRD，SEM and other characterization techniques，and the mechanism of the reaction process was revealed.The test results showed that in the two-step synthesis process，the rod-like magnesium carbonate trihydrate was prepared firstly，which not only improved the filtration efficiency but also greatly reduced the chloride ion entraining rate in the filter cake.Compared with the one-step synthesis process，the chloride ion entraining content in the obtained solid sample was reduced from 6.12% to 2.16%（mass fraction） without using the leach solution.At the same time，the water consumption of the production was only 40% that of the one-step process.The content of MgO and Cl⁻ in the final product met the HG/T 2959—2023 standard of "Industrial hydrated basic magnesium carbonate" and the final yield could reach 97.06%.Therefore，the use of two-step synthesis of basic magnesium carbonate could obviously improve the problem of a large amount of water in the one-step process，and this process could provide an important reference for the preparation of basic magnesium carbonate from the old brine of Qarhan Salt Lake.