The world’s largest sparry magnesite deposit belt is located at the third Member of Dashiqiao Formation of Liaohe Group in Liaodong Peninsula. It is still controversy that these deposits were formed by Mg- rich parasyngenetic brines (2. 1±0. 1 Ga) altered the carbonate precursor to magnesite in an evaporative shallow- marine setting or, epigenetic metamorphic hydrothermal metasomatism (1.85±0.05 Ga). Based on summarizing the geological features and published data, stable isotopes of B and Mg were carried out on the critical geological samples of Haicheng- Dashiqiao magnesite deposit belt. Geological observation and geochemical data shows: ① Though the magnesite deposits are strictly hosted in the third Member of Dashiqiao Formation, the ore thickness shows significant variation among different deposits (30~300 m); The contact relationship between the magnesite ores and its wall rocks of dolomites are mainly performed by faults, with metasomatic magnesite veins constrainted at the margins (5~10 m) of orebodies; the chloritization slates and carbonaceous phyllites are commonly occurred in the ductile share zone of magnesite orebodies, which indicate that the third member of Dashiqiao Formation endured intense tectonic deformation during the late stages; ②Both of the sparry magnesite and dolomite exhibited depleted characteristics of large ion lithophile elements (e.g. K, Rb, Ba, Sr) and "negative shift" of oxygen isotope (δ18O：24.9‰~-11.0‰); As controlled by hydrothermalism, the ∑REE values of carbonates show positive relationship to the content of SiO2; ③Though endured amphibolite facies metamorphism, the magnesite ores inherited the stable isotopic characteristics(δ13C: -2.7‰~+1.6‰; δ34S: +15.6‰~+23.7‰; δ26Mg: -1.53‰~-0.49‰; δ11B: +9.8‰~+15.3‰) of evaporative shallow- marine sediment; ④The fracture- controlling talc ores show relatively heavy magnisium isotopic composition (-0.32‰~-0.15‰), with the light boron isotopic compositions of talc- magnesite(δ11B: +7.7‰~+8.4‰) and leptynite(δ11B: +4.3‰), indicated an isotopic fractionation of hydrothermalism that caused by metamorphism. Therefore, the ore- forming element enrichment of Haicheng- Dashiqiao magnesite deposit belt of Liaodong peninsula is resulted from Mg- rich parasyngenetic brines(2.1±0.1 Ga) altered the carbonate precursor to magnesite in an evaporative shallow- marine setting, rather than epigenetic metamorphic hydrothermal metasomatism.