Strains from Trichoderma reesei have been used for cellulase production with a long history. It has been well known that cellulase biosynthesis by the fungal species is controlled through regulators, and elucidation of their regulation network is of great importance for engineering T. reesei with robust cellulase production. However, progress in this regard is still very limited. In this study, T. reesei RUT-C30 was transformed with an artificial zinc finger protein (AZFP) library, and the mutant T. reesei M2 with improved cellulase production was screened. Compared to its parent strain, the filter paper activity and endo-beta-glucanase activity in cellulases produced by T. reesei M2 increased 67.2% and 35.3%, respectively. Analysis by quantitative reverse transcription polymerase chain reaction indicated significant downregulation of the putative gene ctf1 in T. reesei M2, and its deletion mutants were thus developed for further studies. An increase of 36.9% in cellulase production was observed in the deletion mutants, but when ctf1 was constitutively overexpressed in T. reesei RUT-C30 under the control of the strong pdc1 promoter, cellulase production was substantially compromised. Comparative transcriptomic analysis revealed that the deletion of ctf1 upregulated transcription of gene encoding the regulator VIB1, but downregulated transcription of gene encoding another regulator RCE1, which consequently upregulated genes encoding the transcription factors XYR1 and ACE3 for the activation of genes encoding cellulolytic enzymes. As a result, ctf1 was characterized as a gene encoding a repressor for cellulase production in T. reesei RUT-C30, which is significant for further elucidating molecular mechanism underlying cellulase biosynthesis by the fungal species for rational design to develop robust strains for cellulase production. And in the meantime, AZFP transformation was validated to be an effective strategy for identifying functions of putative genes in the genome of T. reesei.