This study investigated the reactions among CO32-, PO43-, NH4+, Mg2+, and Ca2+, under different CO32- concentration and Mg2+/Ca2+ ratio, and conducted sludge anaerobic digestion (AD) with silicate addition to achieve in-situ CO2 sequestration and nutrients removal. High CO32- concentration facilitated the formation of MgNH4PO4, and Mg2+/Ca2+ ratio of 1:1 achieved best CO32-, PO43-, and NH4+ removal in simulated anaerobic digestate. Supplementation of 40 g/L magnesium silicate combined with 20 g/L wollastonite decreased CO2 content in biogas from 28.2% to 19.0%, and removed PO43- and NH4+ by 61.8% and 21.2%, respectively, in AD. Simultaneous in-situ CO2 sequestration and nutrients removal was achieved by directed precipitation of PO43-, NH4+, and CO2 with silicate released Mg2+ and Ca2+, to form MgNH4PO4 and CaCO3. Meanwhile, methane production was improved by 51.2% with silicate supplementation. This study provides an attractive measure for CO2 and nutrients removal as well as methane production enhancement of sludge AD.