The invention relates to a Fudosteine oxide impurity (R)-2-amino-3-(3-hydroxypropyl sulfonyl) propionic acid (formula I shown in the specification) and a preparation method thereof. The pharmacological activity of Fudosteine (formula II shown in the specification) comes from closed sulfydryl in structure, which is metabolized into a free mercapto derivative in vivo to play a role. If sulphur atom is oxidized, normal metabolism can not be carried out in vivo to obtain the pharmacological activity. Therefore, existence of the impurity (formula I shown in the specification) has a strong impact on medicine quality and medication safety of Fudosteine and needs key control. The specific structure of the oxide impurity is as follows.