[Background] Trichoderma reesei is the most representative fungus to produce cellulase. Epigenetic regulation is a heritable change that does not involve changing DNA sequence. Histone deacetylation is one of epigenetic regulation, and histone deacetylase (HDAC) is responsible for deacetylation. Knocking out the acetylase gene can cause a series of changes in spores, hyphae and cellulase activities. [Objective] A mutant of T. reesei Δhdac was developed by knocking out the histone deacetylase (hdac) gene to investigate the effects of hdac on regulation of cellulase expression. [Methods] The hdac knocking-out expression boxes were constructed by the Split-Maker technique, and were transformed into Trichodermium reesei QM9414. After being verified by PCR and Southern blotting, filter paper enzyme activity and carboxymethyl cellulose natriuretic enzyme activity of the mutant were continuously detected for 7 days, and the related gene expressions were detected by RT-qPCR. [Results] The two enzyme activities in mutant strains were 8.00 IU/mL and 30.00 IU/mL higher than those of the start strain Trichodermium reesei QM9414 respectively. The transcription level of cbh1, egl1 and xyr1 in the T. reesei Δhdac were 6.50 times, 6.01 times and 4.51 times higher than those of the start strain, respectively. [Conclusion] The cellulase gene expression in Trichoderma reesei QM9414 was significantly regulated by the histone deacetylase, this provides a new way to study the effects of epigenetic regulation of Trichoderma reesei on cellulase.