The invention discloses a method for measuring content and particle size distribution of magnetic iron in steel slag. The method comprises the following steps of: performing grading and magnetic separation for the steel slag according to the particle sizes of 10mm, 5mm, 1mm and 0.25mm, and by taking the magnetic separation tails of which the particle size is less than 0.25mm as comparative reference, measuring content of magnetic iron in fine slag of each particle size grade through a specific gravity method, thereby obtaining content and particle size distribution of magnetic iron in steel slag. The method has the characteristics of accurate measurement, wide application range and low measuring cost, and needs no empirical formula. The method is especially suitable for magnetic iron grade measurement of metallic iron recycled by steel slag magnetic separation, and technology setting of graded magnetic separation production, has stronger promotional value, and is beneficial for steel slag resource recycling.