[Background] Due to the low-temperature at maize straw returning time, the straw degradation is slow in the northern part of China. The acceleration of straw decomposition is thus important. [Objective] To isolate strains with high cellulose degradation ability at low temperature from soil sample collected from cold region. [Methods] Dilution and plating method was used to isolate strains from soil sample at low temperature culture. The sodium carboxymethyl cellulose (CMC-Na) hydrolysis circle assay, extracellular enzyme activity assay and straw weight loss method were used to screen fungi with cellulose degradation ability at low cultural temperature. Strains were identified based on method of morphological characteristics and ITS rDNA sequence analysis. Using the reducing sugar method of DNS, the cellulase activity of the strains under different inoculation volumes, initial pH of the medium and culture temperature was studied. The straw degradation ability of the strain was studied by sandbag method. [Results] A cold adapted fungus SDF-25 with higher cellulose activity was screened and identified as Penicillium oxalicum. It could grow from 4 to 37 °C. The optimal cultural condition for cellulase production was determined as follow: inoculation quantity 2%, initial pH 7.0, the optimal temperature 10 °C. The enzyme activity at the optimal cultural condition was 993.3 U/mL. The straw degradation rate of SDF-25 was 39.5% and 44.9% with cultural temperature at 10 °C and 16 °C respectively. [Conclusion] P. oxalicum SDF-25 is a fungus with strong cellulase production ability at low temperature and has a good application prospect in straw returning.