Trichoderma reesei RUT-C30 is a well-known high-yielding cellulase-producing fungal strain that converts lignocellulose into cellulosic sugar for resource regeneration. Calcium is a ubiquitous secondary messenger that regulates growth and cellulase production in T. reesei. We serendipitously found that adding Sr to the medium significantly increased cellulase activity in the T. reesei RUT-C30 strain and upregulated the expression of cellulase-related genes. Further studies showed that Sr supplementation increased the cytosolic calcium concentration and activated the calcium-responsive signal transduction pathway of Ca-calcineurin-responsive zinc finger transcription factor 1 (CRZ1). Using the plasma membrane Ca channel blocker, LaCl, we demonstrated that Sr induces cellulase production via the calcium signaling pathway. Supplementation with the corresponding concentrations of Sr also inhibited colony growth. Sr supplementation led to an increase in intracellular reactive oxygen species (ROS) and upregulated the transcriptional levels of intracellular superoxide dismutase (sod1) and catalase (cat1). We further demonstrated that ROS content was detrimental to cellulase production, which was alleviated by the ROS scavenger N-acetyl cysteine (NAC). This study demonstrated for the first time that Sr supplementation stimulates cellulase production and upregulates cellulase genes via the calcium signaling transduction pathway. Sr leads to an increase in intracellular ROS, which is detrimental to cellulase production and can be alleviated by the ROS scavenger NAC. Our results provide insights into the mechanistic study of cellulase synthesis and the discovery of novel inducers of cellulase.