The effects of spraying different concentrations of lime sulphur,naphthylacetic acid (NAA),ethephon or 3,5,6-TPA at different stages (full bloom,one week after full bloom,two weeks after full bloom and the middle of second physiological drop of young fruits) on flower and fruit thinning of Ponkan (Citrus reticulate Blanco) trees were studied. The results showed that spraying lime sulphur 1-3 °Bé at full bloom and 2-3 °Bé at one week after full bloom had significant effect on flower and fruit thinning.The fruit setting rate was only about half of the control. Either at one week or two weeks after full bloom,spraying three different concentrations of NAA had significant thinning effect with less than 6.5% fruit setting. Similarly,spraying different concentrations of ethephon at three different stages (full bloom,one week or two weeks after full bloom) had also significant effect on flower and fruit thinning,resulting less than 5.5% fruit set. Spraying 10 mg/L of 3,5,6-TPA at the middle of second physiological drop of young fruits did not have any significant effect on flower and fruit thinning. However,the treatment of 3,5,6-TPA at concentrations of 20,30 and 40 mg/L had more significant effect on thinning,the fruit setting rate was only about 1%. Taking the data of this study as a reference,the appropriate chemical reagents and their spraying period and concentration can be selected according to actual needs in the production,and some flowers and fruits can be eliminated to achieve the purpose of replacing the manual sparsely blooming and fruiting and increasing the fruit size.