Magnesium oxy sulfate cement（MOSC），as a new environmentally friendly building material，has the characteristics of early strength and lightweight，but it has problems such as low strength and poor water resistance，which restricts its widespread application.Metakaolin（MK） is formed by the calcination of coal bearing kaolin（coal gangue） and exhibits high volcanic ash activity due to its amorphous structure.In order to compensate for the strength and water resistance defects of MOSC while achieving environmental protection and energy conservation，the effect of MK dosage on MOSC performance based on 1% citric acid modification was explored.XRD and SEM analysis methods were used to analyze the phase composition and microstructure characteristics of the hydrated samples，and the mechanism of MK dosage on cement samples was explained.The results showed that the flowability and setting time of MOSC were decreased with the increase of MK dosage，and the mechanical properties showed a trend of first increasing and then decreasing.From the perspective of mechanical strength and water resistance，the optimal dosage of MK was 10%.The sample cured for 28 days had the best compressive strength of 105.77 MPa and flexural strength of 28.62 MPa，and the best softening coefficient was 0.927.Needle shaped 517 crystal phase appeared in all samples doped with citric acid.The addition of MK promoted the growth and development of 517 crystal phase in the system，and the aspect ratio of 517 crystal phase increased significantly，resulting in a denser system structure.Therefore，the mechanical strength and water resistance of MOSC were significantly improved.