We aimed to investigate the efficacy of carboxymethyl cellulose (CMC) in inducing C. laurentii to antagonize postharvest penicillium (Penicillium expansum) and green mold (Penicillium digitatum) in ‘Shatangju’ mandarin. In this study, the culture time of C. laurentii induced by 0.5% CMC (mass fraction) was optimized for improved antagonistic effect against blue and green mold in commercially available ‘Shatangju’ mandarin. Using sterile water as a control, the effect of C. laurentii cultures induced and not induced by 0.5% CMC on the resistance to blue and green mold, the induction of disease defense system formation and the storage quality of ‘Shatangju’ mandarin was explored. The results showed that the optimal induction time for C. laurentii against both molds was 72 h. The biocontrol of C. laurentii against the two molds was significantly better at 72 h than at other induction times (P < 0.05). C. laurentii induced by 0.5% CMC for 72 h significantly inhibited the incidence and lesion diameter of blue and green mold (P < 0.05). In addition, it increased the activities of β-1,3-glucanase, chitinase, phenylalamine ammonia lyase and polyphenoloxidase and the contents of total phenols and flavonoids in citrus fruit on the eighth day of storage compared to the control group, thereby maintaining the nutritional quality of‘Shatangju’ mandarin during storage. Accordingly, it was concluded that the addition of 0.5% CMC to the culture medium can improve the biocontrol efficacy of C. laurentii. The CMC-induced culture of C. laurentii can increase disease defenserelated enzyme activities, promote the synthesis and accumulation of antioxidant substances in ‘Shatangju’ mandarin, inhibit the occurrence of postharvest blue and green mold, and consequently maintain the storage quality of the fruit.