Soil pretreatment parameters play significant impacts on the accuracy of ion-selective electrode (ISE) in direct soil slurry detection. In this paper, the average relative error (RE) was used to evaluate the influences of three main pretreatment factors, including mesh size, shaking time, and settling time. First, the range of three factors was determined based on the single factor experiment. And then, a 3level, 3-factor Box Behnken experimental design was conducted to investigate their effects. The relationship between RE and three factors was quantitatively described by a multivariate quadratic model through the response surface method. The results showed that settling time was the most influencing one among three tested factors. Mesh size was less significant than shaking time. Coupling settling time and mesh size, the optimal pretreatment parameters were obtained to minimize the detection error, the mesh size was 2mm, settling time was 1.4h, and shaking time was chosen to be 22min. Under the optimized parameters, the RE of the validation soil group were all less than 7.5%, which indicated the prediction accuracy, and the predicted results had a high fitting degree with the measurements. In all, the model was promising for accuracy prediction and analysis in the ISE-based soil slurry measurement.