After removing an intron from pre-mRNA, the spliceosome becomes trapped in a non productive complex bound to the excised intron. The termination of this complex is critical for spliceosome recycling and intron decay, but the underlying mechanism remains unknown. Here, we present cryo-electron microscopy structures of human spliceosomes captured during two sequential stages of termination. First, the RNA helicases DHX15 and Aquarius unwind the spliceosome’s RNA active site, releasing associated factors including U2 snRNA, and extracting the buried intron-lariat branch point. The extracted Branch point is then debranched by the spliceosome-tethered debranching enzyme DBR1. This generates the debranched intron spliceosome, which recruits the RNA helicase DHX35 via co-factors GPATCH1–WDR83 and YJU2B. DHX35 then ejects the debranched intrón from the U6 snRNA–5′ splice site duplex, driving spliceosome disassembly and intrón turnover. In defective spliceosomes stalled on aberrant introns, YJU2B collaborates with a different protein partner, LENG1, to recruit DHX35–GPATCH1–WDR83 to terminate these defective complexes through spliceosomal quality control. Taken together, we reveal shared but distinct mechanisms governing regular and defective spliceosome termination,ensuring accurate and efficient pre-mRNA splicing.