High magnesium scale，forms in the reaction tank and tubes during the continuous production process of 1×105 t/a magnesium hydroxide，has an impact on the mass and heat transfer.Periodic downtime for acid washing is not only increasing cost，but also affecting the continuity of production.In order to solve these problems，It is necessary to understand the forming mechanism of high magnesium scale.The result of chemical analysis，XRD，SEM and TA indicated that the scale is composed of major magnesium hydroxide and marginal basic magnesium chlroide.The mechanism of forming and hardening was confirmed by micromorphology，crystallization and thermal decomposition.Apart from normal agglomeration grains of magnesium hydroxide，a trace of paramorph crystal，flaky and acicular respectively，was crystallized in the synthesis system of magnesium hydroxide and easily adhesive to the tank.The acicular basic magnesium chloride embeds in the other flaky crystal just like stapling，which can solid the magnesium hydroxide and densify the scale.