This paper investigated the influence of steam explosion pretreatment with or without acid as the catalyst on the chemical composition and sugar contents of corn stalks. The fiber characteristics of the pretreated corn stalks were analyzed with a scanning electron microscope, a FS-300 automatic fiber analyzer and a fully automatic surface and pore analyzer. The results showed that the steam explosion pretreatment did not change the cellulose content of the corn stalks. However, hemicellulose was degraded and a portion of lignin was solubilized in the steam explosion pretreatment process. When acid was added in the steam explosion process, the fiber surface and cell wall structure of corn stalks were damaged, the specific surface area and pore size increased, and fiber length decreased, all of which were beneficial to subsequent enzymatic hydrolysis with cellulase. However, content of polysaccharides decreased after acid steam explosion pretreatment.