To establish a rapid and accurate on-line quality control method for aqueous solution of Flos Lonicerae Japonicae extraction (ASFLJE) in the preparation of Qingkailing injection, the wavelength of UV original spectrum and its first derivative spectrum of ASFLJE were scanned. The most suitable spectrum type and wavelength correlated with the chlorogenic acid content in ASFLJE determined by RP-HPLC were chosen to build a regress model. The UV equation for predicting chlorogenic acid content in ASFLJE was established. Other 10 batches of ASFLJE were used for testing the on-line prediction results by comparison of the values of chlorogenic acid content determined by HPLC and predicting equation respectively. The most suitable wavelengths correlated with chlorogenic acid content were 294 nm (r = 0.991 9, n = 28) in the original UV spectrum and 316 nm (r = 0.995 9, n = 28) in the first derivative spectrum respectively. The predicting equation of chlorogenic acid content was: c(mg x mL(-1)) = 506. 254 3 x A316 nm + 0.177 1 (r = 0.995 9, n = 28). This predicting equation is credible and accurate, as tested by HPLC method (r = 0.991 5, n = 10). The prediction results of chlorogenic acid content were: the chlorogenic acid content in probable 90 percent ASFLJE was: 0.4-4.0(mg x mL(-1)). This method is rapid, simple and accurate, and can be used for the on-line quality control of ASFLJE in Qingkailing injection preparation.