Abstract: Accurate determination of the COD_Cr content in activated sludge is essential for resource utilisation. This study proposes an ultrasonic cracking assisted wet method to detect COD_Cr content in activated sludge. Based on the single-factor experiment and using the COD_Cr content as the response value, the response surface method is employed to investigate the individual and interactive effects of three factors:the dilution factor of the sludge mixture, ultrasonic action time, and ultrasonic power, and an evaluation for the influencing factors is conducted. The results show that when the sample volume is 10 mL, the dilution ratio is 225-fold, the ultrasonic action time is 18 minutes and the ultrasonic power is 720 W, the relative standard deviation (RSD) of the samples determination in parallel is less than or equal to 2.00 %, and the spiked recovery rate is between 96.51 % and 105.32 %. The accuracy and precision of the determination results are high. The results of optical microscope and laser particle size analyzer show that ultrasonic can effectively break the floc structure of biological sludge, significantly reduce the size of sludge particles, and reduce the error in the process of sludge sampling. For the sludge samples with different f, the simulated value R² ≥ 0.9961 in the correlation fitting curves between the COD_Cr determination value and the theoretical value. It is proven that the ultrasonic cracking assisted wet method for the determination of COD_Cr content in activated sludge is a method with good reliability, economy and ease of operation.