Plants sense photoperiod signals to confirm the optimal flowering time. Previous studies have shown that Cryptochrome2(CRY2) functions to promote floral transition in the long-day plant (LDP) Arabidopsis; however, the function and molecular mechanism by which CRY2regulates floral transition in short-day plants (SDPs) is still unclear. In this study, we identified a CRY2homologous gene, ClCRY2, from Chrysanthemum lavandulifolium, a typical SDP. The morphological changes in the C. lavandulifoliumshoot apex and ClFTsexpression analysis under SD conditions showed that adult C. lavandulifoliumcompleted the developmental transition from vegetative growth to reproductive growth after eight SDs. Meanwhile, ClCRY2mRNA exhibited an increasing trend from 0 to 8 d of SD treatment. ClCRY2overexpression in wild-type (WT) Arabidopsisand C. lavandulifoliumresulted in early flowering. The transcript levels of the CONSTANS-like(COL) genes ClCOL1, ClCOL4, and ClCOL5, and FLOWERING LOCUS T(FT) homologous gene ClFT1were upregulated in ClCRY2overexpression (ClCRY2-OE) C. lavandulifoliumunder SD conditions. The transcript levels of some circadian clock-related genes, including PSEUDO-REPONSE REGULATOR 5(PRR5), ZEITLUPE(ZTL), FLAVIN-BINDING KELCH REPEAT F-BOX 1(FKF1), and GIGANTEA(GI-1and GI-2), were upregulated in ClCRY2-OE C. lavandulifolium, while the expression levels of other circadian clock-related genes, such as EARLY FLOWERING 3(ELF3), ELF4, LATE ELONGATED HYPOCOTYL(LHY), PRR73, and REVEILLE8(RVE8), were downregulated in ClCRY2-OE C. lavandulifoliumunder SD conditions. Taken together, the results suggest that ClCRY2promotes floral transition by fine-tuning the expression of circadian clock-related gene, ClCOLs and ClFT1in C. lavandulifoliumunder SD conditions.