It has been extensively recognized that the water-to-cement (w/c) ratio, solid content, and particle characteristics of tailings have substantial effects on the fluidity behaviour of cemented paste backfill (CPB). However, it is difficult to evaluate the combined effects of these influential factors. To overcome such limitation, packing density and film thickness theories were introduced in this study. A series of fresh CPB mixes with various binder dosages and solid contents were produced for packing density, flow spreading, flow rate and water film thickness (WFT) tests. The obtained results show that the packing density of solid particle (cement and tailings) system decreases with the increase of binder dosage due to the increase in loosening and wedging effect and the decrease in the particle size distribution width. Higher solid content results in an evident decrease in the water film thickness of fresh CPB, while the influence of binder dosage is not clear. For a given range of specific surface area for solid particles, packing density affects the fluidity of fresh CPB through excess water ratio. The flow spread diameter of fresh CPB increases linearly with higher excess water ratio and WFT. It is also found that WFT is the most important parameter that contributes to the fluidity of fresh CPB. These findings have demonstrated that the packing density and WFT can be reliably used for the estimation of the fluidity of fresh CPB. (C) 2019 Elsevier B.V. All rights reserved.