NOVELTY - 3-(4-Amino-1-oxo-1,3-dihydro-2H-isoindol-2-yl)-2,6-piperidine-dione polymorph (I) having X-ray powder diffraction pattern with first endothermic peak at 169 plus minus 4 degrees C and second endothermic peak at 269 plus minus 4 degrees C, is new.    USE - (I) is useful for preparing drug composition (claimed).    ADVANTAGE - (I): has good storage stability and fluidity. The preparation method is simple and efficient; is environment-friendly and suitable for large scale production.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are also included for:    (1) preparation of (I);    (2) 3-(4-amino-1-oxo-1,3-dihydro-2H-isoindol-2-yl)-2,6-piperidine-dione polymorph (II) having X-ray powder diffraction pattern with peaks (at 2 theta values) as given the specification;    (3) preparation of (II), comprises (a) dissolving 3-(4-amino-1-oxo-1,3-dihydro-isoindol-2-yl-2H)-2,6-piperidine-dione salt in a mixed solvent of deionized water and an organic solvent at room temperature, (b) adding alkali solution into the resulting solution to adjust the pH to 7-9, (c) heating the reaction mixture from 0 degrees C to refluxing temperature while stirring for 0.5-4 hours, and then crystallizing to obtain (II);    (4) 3-(4-amino-1-oxo-1,3-dihydro-2H-isoindol-2-yl)-2,6-piperidine-dione polymorph (III) having X-ray powder diffraction pattern with peaks (at 2 theta values) as given the specification;    (5) preparation of (III), comprises (a) dissolving 3-(4-amino-1-oxo-1,3-dihydro-isoindol-2-yl-2H)-2,6-piperidine-dione salt in a mixed solvent of deionized water and alcohol solvent, (b) adding alkali solution into the resulting solution to adjust pH to 7-9, (c) heating the reaction mixture from 0 degrees C to reflux temperature and stirring for 5-8 hours, and then crystallizing to obtain (III);    (6) 3-(4-amino-1-oxo-1,3-dihydro-2H-isoindol-2-yl)-2,6-piperidine-dione polymorph (IV) having X-ray powder diffraction pattern with peaks (at 2 theta values) as given the specification; and    (7) preparation of (IV), comprises (a) dissolving 3-(4-amino-1-oxo-1,3-dihydro-isoindol-2-yl-2H)-2,6-piperidine-dione salt in a mixed solvent of deionized water and an alcohol solvent, (b) adding alkali solution into the resulting solution to adjust pH to 7-9, (c) heating the mixture from 0 degrees C to refluxing temperature and stirring for 9-24 hours, and then crystallizing to obtain (IV).