In order to quickly and accurately predict the water quality of the Laoha River, the monitoring data of the Laoha River for the period from 2011 to 2015 was used to supplement the missing values using the Lagrange interpolation method. The values of chemical oxygen demand, biochemical oxygen demand, permanganate index, and total phosphorus concentration were respectively used to establish a Levenberg-Marquardt optimized double hidden layer BP neural network model.By using the 2011-2014 data, a training network was established, and the validation and test were conducted with the data of 2015. The results show that: for the five-day BOD forecasting model, when the first hidden layer node number is 4 and the second hidden layer node number is 12, the determination coefficient is 0.751 6 (P=0.000 3), and the average relative error is 25.73%. For the chemical oxygen demand forecasting model, when the number of nodes in the first hidden layer is 12 and the number of nodes in the second hidden layer is 10, the coefficient of determination is 0.887 5 (P<0.000 1), and the average relative error is 27.69%. For the permanganate prediction model, when the number of nodes in the first hidden layer is 6, and the number of nodes in the second hidden layer is 3, the coefficient of determination is 0.854 7 (P<0.000 1), and the average relative error is 28.90%. For the TP prediction model, when the number of nodes in the first hidden layer is 12, and the number of nodes in the second hidden layer is 12, the coefficient of determination is 0.889 2 (P<0.000 1), and the average relative error is 17.94%. The relative error of the BP neural network model of double hidden layer established by complementing the missing data using Lagrange interpolation method is less than 28.90%. The prediction effect of the model is good, and the total phosphorus concentration has the best prediction effect. Through Lagrange interpolation, a dual-hidden artificial neural network model can be established to predict the water quality in the Laihe River Chifeng segment.