【Objective】The present experiment was conducted to elucidate the physiological mechanism of sugar accumulation and changes in photosynthetic parameters in melon fruits during different growth stages in order to improve the quality fruit and its cultivation practices. 【Method】The leaf photosynthetic characteristics and soluble sugar content in fruits of water melon varieties Fengmi 2 and Haoyun 52 were investigated at different stages. 【Result】At the initial stage （April 20） of fruit enlargement， soluble sugar content and reducing sugars in both the varieties were very low， while the photosynthetic and transpiration rates were highest. After 22 d， the total sugar content in both the varieties increased dramatically， and the varieties Fengmi 2 and Haoyun 52 recorded maximum as 13.89 and 10.37% on 5 and 8 June， respectively， meanwhile the net photosynthetic rate and transpiration rate fell down to the minimum. 【Conclusion】The turning point of sucrose accumulation in melon fruits was found to be 22 d after pollination， and net photosynthetic and transpiration rate decreased with the increasing sucrose accumulation and maturation of the fruits. The best harvest time for the varieties Fengmi 2 and Haoyun 52 was 45 and 48 days after pollination with 13.89 and 10.37% of highest total soluble sugar content， respectively.