High-temperature aquifer thermal energy storage (HT-ATES) is regarded as a promising technology for mitigating the mismatch between renewable energy supply and demand, and its performance is strongly governed by formation depth. This study employed three-dimensional thermo-hydro-mechanical coupled simulation and spatial moment quantification to examine depth (600-1400 m) effects on a conventional doublet HT-ATES system. The results show that formation depth significantly affected the extent of energy dispersion: the horizontal dispersion range of thermal and cold plumes at 1400 m was 3.5%-4.0% greater than that at 600 m, with vertical dispersion constrained by thickness. The injection/production pressure difference decreased with formation depth and gradually converged after long-term cycling. The production temperature exhibited the characteristic that deep formations provided superior heat storage capacity, whereas shallow formations demonstrated better cold storage performance. The influence of formation depth on cold well temperature was markedly stronger than that on the hot well, and this disparity intensified progressively with increasing depth. Energy recovery efficiency increased with cycling but decreased with depth, with depth-dependent gaps narrowing. Depth governed system operation via fluid properties, providing a quantitative basis for HT-ATES depth selection, optimization, and long-term stability.