The aim of the research was to study the effects of two plant growth regulators on flower bud differentiation and endogenous hormone contents and provide evidence for the regulation of common buckwheat. PP₃₃₃ and 6BA was sprayed at the different seedling stages on two varieties of ‘BZS’ and ‘CT1’. The bud differentiation was observed by SEM. The flowering time were observed by morphological observation. It recorded the number of flowers and grains. In the process, the changes of three endogenous hormones (IAA, GA₃, ABA) and 6BA were detected by LC/MS. The results demonstrated that: (1) at the second true leaf stage, spraying 100 mg·L^-1 PP₃₃₃ and 150 mg·L^-1 6BA significantly improved the seed setting rate and yield per plant in compare with the control. The seed setting rate both treatments were increased by 49.5%, 39.4%, yield per plant increased by 41.8%, 23.0%, respectively. (2) Flower bud differentiation was divided into five stages, namely, growth primordial undifferentiating period; growth primordial differentiating period; floret primordial differentiating period; stamen and pistil primordial differentiating stage; stamen and pistil primordial formation period. At the second true leaf stage, PP₃₃₃ and 6BA could accelerate the flower bud differentiation process, making the flowering time advance. (3) Spraying PP₃₃₃ reduced IAA and 6BA contents at seedling stage and budding stage, increased GA₃ and ABA contents, but showed the opposite at flowering stage. Spraying 6BA increased GA₃ and IAA contents at seedling and budding stage, reduced GA₃ and IAA contents at flowering stage, reduced ABA content and increased 6BA content at all three stages. The results showed that spraying 100 mg·L^-1 PP₃₃₃ and 150 mg·L^-1 6BA at the second true leaf stage had the best effect, accelerated the flower bud differentiation process, advanced the flowering time, increased the number of seed bearing per plant, and improved the seed setting rate and yield per plant of common buckwheat, clearly proved that the second true leaf stage was the critical period that affected flower development of common buckwheat. Spraying PP₃₃₃ could increase fruiting ability by reducing the number of floweres and improving the ability of vulnerable. 6BA treatment could increase the fruiting by increasing the number of flowering and increasing the number of effective fertile flowers.