A battery of spliceosome-associated proteins has been identified in microRNA (miRNA) biogenesis; however, the underlying mechanisms remain elusive. The intron lariat spliceosome (ILS) complex is highly conserved among eukaryotes and its disassembly marks the end of a canonical splicing cycle. In this study, we show that two conserved disassembly factors of the ILS complex, ILP1 and NTR1, positively regulate microRNA biogenesis through facilitating transcriptional elongation in Arabidopsis. ILP1 and NTR1 form a stable complex and co-regulate alternative splicing of more than a hundred genes across the genome including the core circadian gene LHY and some pri-miRNAs. Dysfunction in either ILP1 or NTR1 result in reduced RNA polymerase II occupancy at elongated regions of MIR chromatins, without affecting MIR promoter activity, pri-miRNA decay and DCL1 processing. Our results provide insights into the molecular mechanisms of spliceosomal machineries in non-coding RNA regulation.