The solid-state fermentation (SSF) of ammoniated corn straw (ACS) by Pleurotus ostreatus Pl-5 was investigated. The SSF experiments were carried out for 20 d using ACS and corn straw (CS) as the substrates of the experimental group (EP) and control group (CP), respectively. The effects of the ammoniation pretreatment on the CS lignocellulose structure, fungal growth, enzyme production, and components of CS during the SSF process were analyzed. The ammoniation pretreatment effectively degraded the lignin and hemicellulose contents in the CS, by 15.3% and 7.7%, respectively. Thus, the in vitro digestibility (IVD) of the EP was higher than for the CP, and even higher than the ligninase activities (laccase: 661 U/g; MnP: 56.8 U/g) found in the CP. The higher cellulase activities (CMCase: 152.3 U/g; FPA: 224.7 U/g) in the EP improved the cellulose degradation, which also promoted the P. ostreatus Pl-5 growth, and the high total N content significantly increased the EP fungal biomass and amino acid contents. A shorter processing time and a higher level of nutrients were achieved by the SSF of ACS, which showed its potential for use in animal feed production.