Abstract: In the future, the output of ultrafine full tailings will explode due to the massive mining of low-grade deposits and demand for the recovery of useful minerals. The best way to dispose of ultrafine full tailings is to prepare them into the paste for filling underground voids or surface stacking. The deep cone thickening of ultrafine full tailings is a key link of tailings paste disposal technology. In the thickening process of ultrafine full tailings, slow sedimentation velocity, high dosage of flocculant, excessive overflow turbidity, and low underflow concentration are the bottlenecks that restrict the application of disposal technology for ultrafine full tailings paste. To investigate the thickening characteristics of ultrafine full tailings, the sedimentation test in measuring cylinder, the small-scale and semi-industrial deep cone dynamic thickening tests were carried out. Results show that the nonionic flocculant with a molecular weight of 12 million is most beneficial for the settlement of the tailings. With increasing flocculant dosage, the turbidity of overflow decreases and underflow concentration remains unchanged. Upon increasing the solid flux, the turbidity of overflow increases and underflow concentration decreases. In particular, when the solid flux is 0.4 t·m⁻²·h⁻¹, the feeding solid mass fraction is 12%, flocculant dosage is 50 g·t⁻¹, and average solid mass fraction of the underflow of the small-scale and semi-industrial dynamic thickening test is 62.8% and 74.4%, respectively. Mud height has a significant influence on the underflow concentration. The solid mass fraction of the underflow of the deep cone thickener increases with the increasing mud height, and this process conforms to the DoseResp function. This growth process can be divided into three stages: (1) slow growth (mud height 1–4 m), (2) rapid growth (mud height 4–7 m), and (3) basic stable (mud height over 7–8 m). This is related to the compression performance of the tailings flocs at different mud heights. According to the function relation between the underflow concentration and mud height, the mud height can be adjusted to meet the required underflow concentration for underground backfilling.