Abstract: A study on the spatiotemporal evolution of the water conducted zone (WCZ) in thick loose layers under a unilateral mined-out area is conducted to overcome challenges such as the complexity of WCZ development and limitations of existing prediction methods. A prediction model for the WCZ development height is proposed, incorporating the pre-damage effect from the existing mined-out area. The research is based on the 866 working face of a coal mine in Huaibei Mining Area. A combined theoretical analysis and field monitoring approach is adopted, and a “working face-coal pillar-adjacent mining area” three-in-one monitoring system for overburden deformation is established using distributed fiber-optic technology. A quantitative determination index for the WCZ development height is formulated based on the gradient distribution of overburden deformation, and a prediction model for the development height of the WCZ under unilateral mined-out area conditions is proposed. Results show that the spatial heterogeneity of the overburden is significant. The subsidence of the mining area exhibits a composite deformation mode dominated by residual subsidence and mining-induced subsidence. The working face area is mainly controlled by dynamic mining disturbance, while the coal pillar zone exhibits stress lag characteristics. The development height of the WCZ is determined by analyzing the inflection point in the cumulative absolute deformation contribution rate curve obtained from distributed fiber optic monitoring, along with the deformation characteristics at various depths. The maximum height is identified as 77 m. After mining stabilizes, the absolute deformation of the rock layers in the WCZ accounts for 87% of the total deformation. A double-zone deformation mode, characterized by upper compression and lower tension, is formed at a depth of −280 m in the existing mining area, causing detachment of the underlying rock layers. A prediction method for the WCZ height that considers the pre-damage effect of the unilateral mined-out area is proposed based on the key stratum theory and slab theory. The overburden is divided into upper and lower rock layers using the key stratum theory, and a failure criterion model is established based on slab theory. The predicted development height of the WCZ for the 866 working face is 71 m, with a relative error of only 8.4% compared to the measured value.