Hydrothermal degradation (HTD) experiments of lawn-grass by the catalytic action of low acid were carried out in the pilot digestor. The effect of the low acid on treated lawn-grass products was investigated in different treatment severities. A severity parameter which combines time and temperature was introduced for comparing results among experiments. The results show that, with the severity increasing, the weight of the solid products, the hemicellulose and cellulose content remarkably decrease; the conversion rate of the organic matter increases; the HTD products by low acid catalysis reach stable in the low severities. At the same time, the soluble sugars and the acetic acid content increase until the treatment severity is over 4.05, however, and furfural content always increases in the liquid products.