Plants can be grouped into four categories: long-day plant,short-day plant,intermediate-day plant and day-neutral plant according to their different reactions to photoperiod.Because the length of darkness is the crucial factor determining plant's flower-bud formation,long-day plant should be named short-night plant,short-day plant should be named long-night plant.The course of transformation from vegetative growth to reproductive growth is termed floral induction.There are four pathways in floral induction such as photoperiod,vernalization,gibberellin and autonomous promoting.Photoperiod pathway is the best studied pathway.This paper introduced the molecular mechanism of plant flower-bud formation induced by photoperiod from three aspects: photoreceptor,biological clock components and some relevant genes.Flowering regulation is feasible by controlling temperature,light,plant growth substance and management.