Cu2ZnSnS4 (CZTS) is a promising photovoltaic absorber material,however,efficiency is largely hindered by potential fluctuation and a band tailing problem due to the abundance of defect complexes and low formation energy of an intrinsic Cuzn defect.Alternatives to CZTS by group Ⅰ,Ⅱ,or Ⅳ element replacement to circumvent this challenge has grown research in-terest.In this work,using a hybrid (HSE06) functional,we demonstrated the qualitative similarity of defect thermodynamics and electronic properties in Cu2MgSnS4 (CMTS) to CZTS.We show SnMg to be abundant when in Sn-and Cu-rich condition,which can be detrimental,while defect properties are largely similar to CZTS in Sn-and Cu-poor.Under Sn-and Cu-poor chemic-al potential,there is a general increase in formation energy in most defects except SnMg,CuMg remains as the main contribu-tion to p-type carriers,and SnMg may be detrimental because of a deep defect level in the mid gap and the possibility of form-ing defect complex SnMg+Mgsn.Vacancy diffusion is studied using generalized gradient approximation,and we find similar va-cancy diffusion properties for Cu vacancy and lower diffusion barrier for Mg vacancy,which may reduce possible Cu-Mg dis-order in CMTS.These findings further confirm the feasibility of CMTS as an alternative absorber material to CZTS and suggest the possibility for tuning defect properties of CZTS,which is crucial for high photovoltaic performance.