Based on the rainfall measurement of 79 rain events and the chemical composition determination of rainwater at Guangzhou's Longdong from November 2003 to October 2004, the acid rain frequency, chemical composition, and its correlation were studied in the paper.The results showed that: (1) Total amount of rainfall was 1359.8 mm during the monitoring period.The pH values varied from 3.22 to 7.29 and with an average of 4.83.The frequency of acid rain (pH<5.6) is 77.2%, and the amount of acid rain accounts for 81.5% of total rainfall.(2) Acid rainwater takes SO₄^2-, Cl^- and NO₃^- as main anion, and has high concentration of Ca²⁺, NH₄⁺ and Na⁺.(3) The acid rain at Guangzhou belongs to sulphate-dominated type.The concentration of SO₄^2-, NO₃^-, PO₄^3-, Cr, NH₄⁺, K⁺, Na⁺, Ca²⁺and Mg²⁺ in rainwater are 0.150, 0.045, 0.002, 0.163, 0.058, 0.006, 0.013, 0.083 and 0.007 mol·L^-1, respectively.The SO₄^2-/NO₃^- ratios vary from 0.42 to 17.6, with an average of 3.33, which is much lower than that in southwest China.And the ratio is also less than that measured 5 years ago in the same place.It indicates that the high NO_x emission at Guangzhou is a severe environmental threat.(4) There is obvious seasonal variation in rainwater chemistry at Guangzhou, it is that acid rain is more serious in spring, summer and autumn but not in winter.That is the reverse of what observed at Chongqing of southwest China, where acid rain being more serious in winter than in spring.(5) According to the SPSS analysis of measured data, there is a negative relation between rainfall and concentration of each of major element composition, which can illuminate the dilution effect of atmospheric pollutants by rainwater.There is also certain correlation among the chemical compositions of acid rain, such as significant positive relationships between SO₄^2- and Ca²⁺, NO₃^- and NH₄⁺, K⁺ and Na⁺/Mg²⁺, Na⁺ and Mg²⁺, respectively..