The ultrasonic attenuation caused by impurities in the water leads to inaccurate measurement of ultrasonic flow meter. Numerical simulation and experimental research were used to study the distribution and measurement accuracy of the impurities in the ultrasonic flow meter. Impurity distribution in liquid was calculated by Eulerian twophase model and standard k-ε model. Through the two-phase flow mathematical model to establish the numerical calculation of the internal flow, obtain the impurity distribution law of different impurity particle sizes under different flow conditions. When the particle size of impurities was less than 0.05mm, it was easier to be carried by the fluid. Impurities were relatively evenly distributed in the flow channel. When the Reynolds number was small, impurities larger than 0.1mm in diameter were distributed below the signal propagation path. As the Reynolds number was increased, the interaction between the solid phase and the liquid phase became stronger, and the impurity concentration in the signal propagation path was increased. Through the experiment, the measurement error of the ultrasonic flow meter of the impurity water flow was studied. The numerical calculation and the experimental results were mutually verified. The influence of different impurity particle sizes on the acoustic propagation center area and the ultrasonic wave were given. Measurement errors caused by impurities smaller than 0.02mm in diameter were more easily corrected. The measurement error caused by impurities with larger particle size was greatly affected by the fluid velocity. The research can provide a theoretical reference for the research of the detection accuracy of ultrasonic flow meter in the actual water quality with different particle impurities, and had a certain application value.