A composition of (6S)-5-methyl tetrahydrofolic acid or a salt thereof as well as the preparation therefor and an application thereof. The content of the (6S)-5-methyl tetrahydrofolic acid or the salt thereof is not less than 98.0%, the content of a related impurity JK12A is not greater than 0.1%, and 5-methyl tetrahydropteroic acid shall not be detected.