[Background] Xylan which widely exists in lignocellulosic biomass is the most abundant hemicellulose in the world. It is an effective way for the resource and energy utilization of lignocellulosic biomass to utilize enzyme producing microorganisms to ferment lignocellulosic biomass. [Objective] The aim of this experiment was to obtain materials for producing multiple feed additives from multi fiber agricultural and forestry wastes by screening and identifying xylanase producing strains, analyzing enzymatic characteristics and optimizing fermentation conditions. [Methods] The xylanase producing strains were screened from the soil of Qinghai-Tibet Plateau. The strain XC70 were identified by morphological observation and rDNA ITS sequence analysis. The enzymatic characteristics, growth and enzyme production attributes of the strain were analyzed, and the fermentation conditions were optimized by single factor method and orthogonal test method. [Results] The strain XC70 was identified as Penicillium oxalicum by morphological and molecular biological methods. The results showed that the optimal reaction conditions of xylanase produced by XC70 were pH 5.0, 70℃, the xylanase had good stability when the temperature was lower than 50℃, and it showed some acid resistance. Na⁺ and K⁺ promoted the xylanase activity (P<0.05). After 54 h of fermentation, the biomass and enzyme activity of supernatant reached the peak. Through single factor method and orthogonal test method, the optimal fermentation conditions were determined as follows:peptone 7 g/L, corn straw 50 g/L, KCl 4 g/L, initial pH 4.0, 28℃, shaking speed 200 r/min, inoculum size 2%. Under these conditions, the xylanase activity reached 1 489.33 U/mL, which was three times higher than that before optimization. [Conclusion] The strain XC70 screened from the soil of Qinghai-Tibet Plateau has certain acidic xylanase-producing capacity, which can be used to degrade multi fiber materials and develop new feed additives, and this has certain application potential and development value.