The present invention relates to a method for preparing acrylic acid from glycerin. More specifically, the present invention provides a method which can improve the selectivity of acrolein by applying a specific catalyst composition and process conditions to minimize the generation of coke carbon of the catalyst, and can prepare acrylic acid with higher productivity for a longer duration of time because a dehydration reaction can be performed for a longer working period while maintaining catalyst activity at a high level during the reaction.