HP?MOF?74(Mg _x Ni_1-x ) (x=0.25, 0.50, 0.75) was synthesized by solvothermal method with Mg and Ni as central metals and sodium dodecylbenzene sulfonate (SDBS) as template. Using flue gas CO₂ and N₂ as adsorbents, the performance of HP?MOF?74(Mg _x Ni_1-x ) in adsorption and separation of CO₂/N₂ at 273 K and 298 K was investigated, the isotherms of CO₂ and N₂ on three different HP?MOF?74(Mg _x Ni_1-x ) materials were measured at 273 K and 298 K by static volumetric method, and the experimental data sets were fitted by Dual Site Langmuir Freundlich (DSLF) and Single Site Langmuir Freundlich (SSLF) models. The adsorption selectivity of CO₂/N₂ binary mixture was estimated according to the Ideal Adsorption Solution Theory (IAST). In addition, the isosteric heat of adsorption (Q_st) was calculated using the Clausius Clapeyron equation. The results show that the CO₂ adsorption capacity of HP?MOF?74(Mg_0.50Ni_0.50) sample is 4.864 mmol/g at 273 K and 100 kPa; the adsorption isotherms of CO₂ and N₂ on HP?MOF?74(Mg _x Ni_1-x ) are in good agreement with Dual Site Langmuir Freundlich (DSLF) and Single Site Langmuir Freundlich (SSLF) models respectively, indicating that the adsorption behavior of CO₂ is double sites adsorption, while the adsorption behavior of N₂ is single site adsorption; at 273 K and 100 kPa, the IAST adsorption selectivity of HP?MOF?74(Mg_0.25Ni_0.75) for CO₂ is 2 263, and the adsorption capacity and selectivity are better than those of traditional adsorbent MOF?74; the CO₂ isosteric heat of adsorption on HP?MOF?74(Mg _x Ni_1-x ) is higher than that of N₂, indicating that the surface free binding energy of CO₂ on HP?MOF?74(Mg _x Ni_1-x ) is higher.