Provided are a crystalline form G of a free base of an imidazo-isoindole derivative and a preparation method therefor. The chemical structure of the compound is shown in formula (I), and the chemical name thereof is (S)-2-(4-(4-(4-(6-fluoro-5H-imidazo[5, 1-a] isoindole-5-yl) piperidine-1-yl) phenyl)-1H-pyrazol-1-yl) ethanol. Provided are the application of the crystalline form G of the compound in a pharmaceutical composition and the pharmaceutical use of the crystalline form G and the composition in the preparation of a medicine for treating diseases with pathological characteristics of tryptophan metabolic pathway mediated by IDO (indoleamine-pyrrole-2, 3-dioxygenase). The crystalline form G of the compound has good stability, and the crystallization solvent that is used has low toxicity and residue and may be used well in clinical treatment.