Objective To establish a method for qualitative identification of polymorphs in pharmaceutical solid preparations of active pharmaceutical ingredients ( API ) . Methods We obtained the powder diffraction patterns of the polymorphic drug substance like nimodipine and roxithromycin in solid preparation material and completed quantitative identification for polymorphs by the quantitative detection and using PXRD technology, deduction calculation through the powder X-ray diffraction and comparing with standard diagram. Results Through the analysis of nimodipine and roxithromycin which came from 27 batches of solid preparations from 11 different manufacturers, and comparing to the standard patterns of pure polymorphs, the quantitative identification of different crystalline states of API in pharmaceutical preparations had been established. Conclusion The qualitative detection method for polymorphs of API in pharmaceutical preparations by powder X-ray diffraction has wide applicability and high accuracy, which can be used to identify the polymorphism of API in solid preparation,and also used to control the quality of solid preparations commonly as a qualitative analysis method.