Abstract: To discuss the stocky efficiency of plant growth retardants, characteristics of betelnut(Areca catechu L.) photosysthesis in seedling stage were studied. Field experiments with Hainan betelnut were conducted with 5 treatments , including 250mg/L and 500mg/L of paclobutrazol(PP333)、40mg/L and 80 mg/L of uniconazole (S3307)、natural water(CK)) fromJune to November in 2006 after transplanted 45 days. The content of chlorophyll and chlorophyll fluorescence parameters of leaf in betelnut were investigated under different plant growth retardant treatments. The results showed that the content of chlorophyll、the potential activities of PSⅡ(Fv/Fo),conversion eficiency of primary light energy of PSⅡ(Fv/Fm) and photochemical quenching coefficient(qP)were distinctly enhanced，but the non-photochemical quenching coeficient(qN) and NPQ (non-photochemical quenching ) were decreased. 250 mg/L of PP333 was obvious. The conclusion was that the optimum PP333 concentration can add the open proportion of PSⅡ reaction center,promote photosynthetic electron transport,enhance the ability of photosynthetic electron transport,promote photostructure,and strengthen the sturdy stem of betelnut.