To enhance the early compressive strength of magnesium oxysulfate（MOS） cement， 5Mg（OH）₂·MgSO₄·7H₂O （517） whiskers after heat treatment were added to the MOS system. The effect of the 517 whiskers with and without heat treatment on the setting time， compressive strength， phase composition， microstructure and pore structure of MOS cement was investigated. The results show that both 5Mg（OH）₂·MgSO₄·4H₂O and 5Mg（OH）₂·MgSO₄ can adsorb water molecules from the MOS slurry to convert into 517 whiskers. This can further promote the formation of 517 whiskers in MOS system due to the adhesive-crystal seed synergistic effect of these two dehydrated phases， resulting in a shortening in the initial and final setting time， an optimization in the pore structure， and an improvement in the early and late compressive strength of MOS cement. In addition， the content of 517 whiskers used to prepare MOS cement should not exceed 4%. Compared with 517 whiskers without any heat treatment， the 517 whiskers treated at 100 ℃ and 150 ℃ have a stronger positive effect on the performance of MOS cement.