First-principles method is used to study the native point defects in Y2SiO5 and Y2Si2O7 silicates. The calculated defect formulation energies show similar native point defect behaviors in Y2SiO5 and Y2Si2O7: the oxygen Frenkel defect is predominant; and it is followed by the cation antisite and Schottky defects. The possible chemical potential range of each constituent is further considered in the calculation of defect formation energy. Oxygen interstitial (Oi) and oxygen vacancy (VO) are the predominant native point defects under O-rich and O-poor condition, respectively. In addition, the mechanisms of accommodating composition deviations from stoichiometric Y2SiO5 and Y2Si2O7 are investigated. For Y2SiO5, Y2Si2O7 impurity may appear, together with the defects of SiY antisite, Oi interstitial, and/or VY vacancy when SiO2 is excess; while YSi antisite appears together with Yi interstitial and/or VO vacancy in Y2SiO5 when Y2O3 is excess. For Y2Si2O7, the main process is the formation of SiY antisite accompanied by Oi interstitial and/or VY vacancy when SiO2 is excess; but Y2SiO5 impurity forms, together with YSi antisite, VO vacancy, and/or Yi interstitial in Y2Si2O7 when Y2O3 is excess. We expect that the results are useful to control of processing conditions and further to optimization of performance of the two silicates.