Abstract: With black jade watermelon as experimental material, the safety of applying rare-earth lanthanum to watermelon was studied. The results showed that in the absence of exogenous application conditions, the content distribution of rare-earth lanthanum in water melon plants was root > leaf > stem > peel > pulp, and the content in pulp is extremely low for 0.065 mg/kg. And the contents of rare-earth lanthanum in pulp were 0.083 mg/kg and 0.070 mg/kg in the two treatments spraying 120 mg/kg and 240 mg/kg respectively before the 7 days of the harvest of watermelon, which content were far lower than the limited standards allowed in fruits, and it illustrated that the application of rare-earth lanthanum to watermelon is safe.