Method for preparing montelukast acid

The invention discloses a method for preparing montelukast acid. The method comprises a reaction b or a reaction a to a reaction b in the following synthetic route: the formula is shown in the description. The method disclosed by the invention can be used for preparing montelukast acid and salts thereof with low cost and high yield by means of simple operations by using low-cost and easily available raw materials, and is of significance and has practical value to prepare montelukast acid and salts thereof with low cost on a large scale.