With tomato cultivar ‘Jinpengchaoguan’ as the experimental material, foliarspraying 0.25 mmol·L^-1 spermidine (Spd) were applied to the seedlings, which grows under high temperature condition (38 ℃/28 ℃, day/night). The effects of foliarspraying Spd on the growth, the contents of chlorophyll precursors, key enzyme activity and the content of chlorophyll of tomato seedlings were discussed. The results showed that: (1) foliarspraying Spd significantly alleviated the decrease of shoot weight of tomato seedlings under heat stress, but it had no significant effect on root weight. (2) The contents of chlorophyll precursors δaminolevulinic acid (ALA) and porphobilinogen (PBG) in tomato leaves increased significantly under heat stress, but the contents of uroporphyrinogen Ⅲ (UroⅢ), protoporphyrin Ⅸ (Proto Ⅸ), Mgproto Ⅸ (Mgproto Ⅸ) and protochlorophyll (Pchl) decreased significantly under heat stress. The results indicated that chlorophyll synthesis blocked site was from PBG to UroⅢ, which might cause the reduce of chlorophyll a (Chla), total chlorophyll (Chlt) and Chla/Chlb. (3) Foliarspraying Spd increased the activity of porphobilinogen deaminase (PBGD) of seedlings under heat stress, which effectively alleviated the hindrance of heat stress on the transformation from PBG to UroⅢ, and promoted the synthesis of chlorophyll precursors after PBG. In summary, this study showed that tomato growth and chlorophyll synthesis were significantly inhibited under heat stress. This inhibition could be relived through foliarspraying Spd, which could alleviate the transformation from PBG to UroⅢ. This process promoted the synthesis of chlorophyll precursors, resulting in increased amount of chlorophyll and less inhibited growth of tomato plants under heat stress.