Efficient deconstruction of lignocellulose is achieved by the synergistic action of various hydrolytic and oxidative enzymes. However, the aldonolactones generated by oxidative enzymes have inhibitory effects on some cellulolytic enzymes. In this work, d-glucono-1,5-lactone was shown to have a much stronger inhibitory effect than d-glucose and d-gluconate on beta-glucosidase, a vital enzyme during cellulose degradation. AltA, a secreted enzyme from Penicillium oxalicum, was identified as an aldonolactonase which can catalyze the hydrolysis of d-glucono-1,5-lactone to d-gluconic acid. In the course of lignocellulose saccharification conducted by cellulases from P. oxalicum or Trichoderma reesei, supplementation of AltA was able to relieve the decrease of beta-glucosidase activity obviously with a stimulation of glucose yield. This boosting effect disappeared when sodium azide and ethylenediaminetetraacetic acid (EDTA) were added to the saccharification system to inhibit the activities of oxidative enzymes. In summary, we describe the first heterologous expression of a fungal secreted aldonolactonase and its application as an efficient supplement of cellulolytic enzyme system for lignocellulose biodegradation.