We selected nine fruit thorns and five fruit non-thorns of castor as the research object using the RAPD marker for the thorn traits analysis. The results were as follows:the RAPD-PCR reaction system with opti-mized by single factor was 0. 9 μL of Taq polymerase,2. 5 μL of Taq polymerase 10 × Buffer,2. 5 μL of each dNTP,1. 3 μL of Mg2+,1. 4 μL of 10 pmol/L primer SBS126,1. 4 μL of DNA,ddH2 O 15. 0 μL,Total 25 μL;And the related amplification condition was 1 cycle of 4 min 30 s at 94 ℃;35 cycles of 45 s at 94 ℃,45 s at 38 ℃and 45 s at 72 ℃,followed by a final cycle of 5 min at 72 ℃,then save in 4 ℃. Using the optimized reaction system and condition,several 1 800 bp fragments amplified by SBS126 was found in all the fruit thorn material,and sequen-cing results showed that the upstream homology of fragment was very low,whereas the downstream has the identical sequence with about 112 bp length. Basing on these results,we deduced that the development of fruit thorn may be associated with histidine-containing phosphotransfer protein 2,and this laid a foundation for research on the molecu-lar mechanism of fruit thorn traits for castor.