In order to explore the effects of high temperature stress on tomato seedling growth and flower bud differentiation, we subjected ‘Zhongza 9’ tomato seedlings to high temperature treatments with 37 ℃/27 ℃ (day/night) when it grew to the critical point of flower bud differentiation, 25 ℃/15 ℃ (day/night) was set as control. The indicators related to the growth and flower bud differentiation of tomato seedlings were determined. The results showed that the plant height and internode length of tomato seedlings were significantly longer than that of the control from the third day of high temperature stress, and the stem diameter was significantly smaller than that of the control, that is, the plants appeared to be long. The leaf area and root surface area of tomato treated with high temperature were always significantly lower than that of normal temperature control, while the root volume, total root length and number of roots were significantly lower than that of control from the 3rd day. The flower bud differentiation process at high temperature treatment accelerated significantly from the 1st day, and the number of flower bud differentiation decreased significantly on the 9th day of high temperature treatment, while the flower bud size was larger than that of control 5 days before high temperature treatment, and the flower bud size of the high temperature treated plant was significantly lower from the 7th day. This study clarified that high temperature inhibited the vegetative growth of tomato seedlings, accelerated the flower bud differentiation process, reduced the number of flower bud differentiation, and reduced the size of flower buds, eventually leading to a decrease in yield.