The reactive crystallization kinetics of hydromagnesite in the MgCl2-CO2-NH3-H2O system, which plays an important role in a new approach to sequester CO2 with saline sea/lake brines, was systematically investigated by the MSMPR (mixed-suspension-mixed-product removal) crystallization method. The temperature effect on the crystallization of magnesium carbonate hydrates in the system was first investigated batchwise. The optimum temperature of 353.2 K for precipitation of the regular spherical hydromagnesite with good filterability was selected for kinetic study. The supersaturation cf hydromagnesite was experimentally tested, and the activity coefficients of aqueous species were strictly calculated by the Pitzer model embedded in the Aspen Plus platform. The nucleation and growth rate and agglomeration kernel were determined by moments analysis based on the PSD (particle size distribution) of crystals in volume coordinates. The corresponding kinetic parameters in three empirical equations were then estimated by linear regression. The resulting volume growth rate order of 2.30 means that surface integration is dominant in the volume growth of hydromagnesite. A simplified process for the new sequestration approach was finally constructed. The value of hydromagnesite as carbonated product was assessed considering the scale-up and costs of such a process.