In early spring，under successive cold，rainy and cloudy climate condition，this experiment took
‘Nabite’ scions grafted onto‘ Yongzhen No.5’ as material，and exposed them under LED light and high voltage
sodium lamp for supplementary light of continuous 25 days，taking natural light as the contrast. The results showed
that artificial supplement light could obviously accelerate the growth of grafted watermelon seedlings. After the
17th day under supplementary light，the artificial supplement light treatments had great impact on the growth of
seedlings：the hypocotyl lengths of rootstock and scion were rapidly increased，the plant height was remarkably
enlarged，leaf areas became bigger，stems became thicker，biomass accumulation was fastened，chlorophyll
content was improved，photosynthetic capacity of leaf blades was strengthened，survival rate of grafted seedlings
was improved，diseases occurrence rate was reduced. Compared with different light resources，the effect of LED
light is better than that of high voltage sodium lamp. Moreover，LED light has lower input and usage cost in long
time running. LED light might become one of the most promising artificial supplementary lights in agriculture field
in the future.