The tuff powder has certain adsorption to the polycarboxylic acid water reducing agent（PCs）, which makes the number of “effective water reducing agent” decrease, resulting in greatly reduced performance of PCs. The anti-adsorption characteristics of PCs have important design reference significance, and it is closely related to the composition and structure of PCs. The UV spectrophotometry is a common method for the detection of adsorption quantity, but there are still many uncertainties in the PCs test. So, UV spectrophotometry and derivative spectrometry were used to quantitatively analyze the adsorption capacity of tuff powder on PCs with different side chain lengths, and the experimental parameters were analyzed and optimized. The results show that there is no obvious ultraviolet characteristic peak in the absorption spectrum of PCs. When test solution concentration is increased and its pH value is lowered, the pseudo peak can be seen in the wavelength range of 190~200 nm, which is confirmed by the spectral test of acetic acid analysis reagent. After the derivative treatmentof the PCs spectral data, the characteristic peak of PCs can be obtained. There is a good linear relationship between the test solution absorbance and its concentration under this characteristic peak, and the correlation coefficient r is greater than 099. Further, the accuracy of UV absorption spectroscopy is demonstrated, compared with the TOC method. There is a good linear relationship between the above two methods, and the correlation coefficient r is 0997, which indicates that second derivative UV spectrum can provide a simple, fast, accurate, inexpensive and non-developing agent quantitative test method for PCs adsorption analysis; The adsorption of the PCs by the tuff rock powder decreases as the length of its side chain decreases. The research results will help to provide a new way for UV detection of organic matter with weak ultraviolet absorption.