This　work　investigated　the　immobilization　of　municipal　solid　waste　incineration　(MSWI)　fly　ash　using　spent　caustic　as　alkaline　activator　and　blast　furnace　slag　(BFS)　as　an　additive,　in　which　comparable　alkalinity　NaOH　solution　was　used　as　controlling　alkaline　activator.　The　leaching　behavior　of　heavy　metals　and　organics　in　alkali-activated　MSWI　fly　ash-based　mortars　were　characterized　by　HJ/T　300–2007　method,　BCR　chemical　speciation　analysis,　and　total　organics　carbon　(TOC)　and　organics　measurements.　It　was　found　that　the　spent　caustic-activated　mortars　exhibited　a　weaker　immobilization　performance　for　Cu,　Pb,　Zn　and　Cd,　and　a　better　performance　for　Cr　(VI),　comparing　to　NaOH-activated　mortars.　The　presence　of　organics　and　the　formation　of　ettringite　in　spent　caustic-activated　mortars　made　the　immobilization　of　Cr(VI)　more　feasible.　As　increasing　the　BFS　dosage,　the　immobilization　efficiency　of　heavy　metals　was　improved.　For　the　immobilization　of　organics,　the　spent　caustic-activated　mortars　were　of　a　medium　performance.　This　study　extends　a　novel　waste-derived　alkaline　activator　in　the　alkaline　activation　filed　and　would　favor　the　development　of　waste　management.　©　2020　Elsevier　Ltd