Abstract: Changes of the organic acid and soluble sugar contents in the fruits of Shuanglong pitaya during the development stage were monitored.The results showed that there were malic,oxalic,citric,tartaric and fumaric acids in the fruits.The total acid content,which changed in an inverted V fashion during the entire fruit development period,peaked at 1.29 g·hg-1 on the 25th day,and decreased to 0.54 g·hg-1 35 days after flowering.Oxalic acid was most prominent during the early development stage(i.e.,6 to 15 d after flowering),whereas,malic acid dominated in the middle and late stages(i.e.,16 to 35 d after flowering).The sugar content accelerated rapidly from 20 to 35 d,and reached its maximum of 12.45 g·hg-1 35 days after flowering.The sugar/acid ratio changed in a similar manner as the soluble sugars,with a peak value of 23 at 35 d after flowering.