U-series isochron technique has been applied to determine the age of impure carbonates that were blended by one natural detritus and a pure CaCO3 with known age in different proportions. The experiment was designed two sets: The first one was to evaluate the extracting behaviour and fractionating effect of isotopes U and Th of detritus during leaching samples. Another set was to examine the effect of acid attacks on both the carbonate and the detritus fractions in a mixture of different proportions when treated with hydrochlority acid of different concentrations. Both the leachates and residuals was analyzed for 238U, 234U, 230Th and 232Th concentrations, which indicated that the detritus dissolved by diluted acid have extracted U and Th isotopes based on constant proportion, thus the isotopic fractionation would not occur. For impure carbonates samples, all data points fell on the straight line, such good linear relationship between the isotopic ratios of the leachate and residue fractions could reflect the meaning of true age. The L/R method was applied to the artificial impure carbonate and the resulting age is found in agreement with the age of the pure carbonate sample, which showed that the technique was reliable. Therefore, for the U/Th age determination of impure carbonates with a higher content of detritus, the L/R isochron dating would be adaptable.