A　prediction　model　was　developed　in　this　paper　to　quantify　the　effect　of　fly　ash　on　the　chloride　diffusion　process　in　concrete　samples.　To　capture　the　hierarchical　characteristics　of　constituents　at　multiple　scales,　the　model　adopted　the　general　self-consistent　(GSC)　method　to　calculate　the　effective　chloride　diffusivity　of　fly　ash　concrete.　Moreover,　the　proposed　model　adopted　the　Papadakis　model　to　estimate　the　amount　of　hydration　products　of　fly　ash　pozzolanic　reaction　as　well　as　cement　hydration.　In　addition,　the　chloride　diffusivity　of　fly　ash　concrete　was　corrected　by　introducing　the　filling　effect　of　unreacted　fly　ash　particles　and　the　aggregate　tortuosity　factor.　Beside　the　theoretical　modelling,　this　study　also　carried　out　RCM　and　RCPT　tests　on　concrete　samples　with　different　replace　ratio　of　fly　ash,　whose　measurements　were　used　to　validate　the　proposed　model.　A　good　agreement　was　obtained　between　model　calculations　and　experimental　measurements　demonstrates　the　proposed　model　can　successfully　characterize　the　effect　of　fly　ash　on　chloride　diffusion　in　concrete　samples,　which　can　be　an　effective　means　to　assessing　the　durability　of　fly　ash　concrete　materials　and　structures　in　practical　engineering.