Provided are a process for preparing a Parp1/2 inhibitor, i.e., (R)-2-fiuoro-10a-methyl-7,8,9,10,10a,11-hexahydro-5,6,7a,11-tetraazacyclohepta[def]cyclopenta[a]fluoren-4(5H)-one (hereinafter referred to as Compound A), crystalline forms (polymorphs) of Compound A or hydrate or solvate thereof, methods for preparing the crystalline forms, and the use thereof.