To fully utilize the engineering waste soil, it is thoroughly mixed with a curing agent and water to prepare a new type of filling material that does not require compaction after backfilling and has good engineering properties. In this paper, depending on a foundation pit fertilizer trench backfill project in Nanjing, direct shear tests were carried out on different proportions of premixed fluid soil to study the shear characteristics of premixed fluid soil. Based on the neural network model, the prediction of the shear stress versus shear displacement curve of premixed fluid soil was carried out. The results indicate that the type of shear stress versus shear displacement curve of premixed fluid soil is influenced by the cement mix ratio, curing age, and vertical pressure. The shear strength and cohesion of premixed fluid soil are influenced by the water cement ratio, vertical pressure, curing age, and cement mix ratio. The neural network prediction model for shear stress versus shear displacement curve of premixed fluid soil has a high accuracy in predicting the shear strength.