Abstract: Deep-cone thickening of unclassified tailings is one of the key technologies in the field of cemented paste backfill. Flocculation and settling behavior of unclassified tailings constitute key research topics of the deep-cone thickening technology. Based on the measurement of the floc chord length during the unclassified tailings flocculation process, this study investigated the flocculation and settling processes independently, which is different from the traditional research. First, the average chord length of the floc was used as the index to study the flocculation behavior of the unclassified tailings under different conditions. Then, the initial settling rate of the suspension–supernate interface was used as the index to analyze the settling behavior of the unclassified tailings slurry under varying flocculation conditions. Under different flocculation conditions, the unclassified tailings particles were flocculated rapidly and the average chord length of the floc increased rapidly to the peak value. Then, it decreased gradually with the shear time until it reached a stable state. It was found that the average chord length of the flocs of the unclassified tailings and the initial settling rate of the suspension–supernate interface of the flocculated, unclassified tailings slurry vary with flocculation conditions. For the scope of this study, the optimal flocculation conditions were determined as follows: the flocculant used was Magnafloc 5250, the solid mass fraction was 10%, the flocculant dosage was 10 g·t⁻¹, the flocculant mass fraction was 0.025%, and the shear rate was 94.8 s⁻¹. Under such flocculation conditions, the peak value of the average chord length of the floc was 620.63 μm, the average chord length after the flocculation was 399.57 μm, and the initial settling rate of the suspension–supernate interface of the flocculated tailings slurry was 4.61 mm·s⁻¹. The initial settling rate model of the suspension–supernate interface, applicable only to the tailings used in this study, was established preliminarily based on the average chord length of the flocs. The initial settling rate of the suspension–supernate interface increased with the increase in the average chord length of the flocs, providing a reference for the control of flocculation and settling parameters and the optimization of the equipment structure to improve the flocculation settling efficiency of unclassified tailings slurry in actual production.