The key parameters for biomass hydrothermal treatment were optimized for product distribution on the basis of a L16 (45) orthogonal experiment design. Results showed that biomass species, particle size and hydrothermal temperature significantly affected heavy oil yield. By contrast, the effect of biomass concentration was negligible. The maximum heavy oil yield was 28.00% at the optimal condition (biomass species, pine sawdust; hydrothermal temperature, 250℃; biomass particle size, 80~150 mesh; residence time, 15 min; concentration, 10 g of raw material and 110 g of water). In addition, cotton straw yielded the most liquid in the agricultural straws, although ash content was low. The cotton straw was treated in an autoclave reactor. The influences of temperature, residence time, catalysts and the size of cotton straw particles on product distribution were investigated. The increase in temperature reduced solid residue yield. Moreover, the heavy oil yield was increased firstly and then decreased. The yields of light oil and gas increased. Specifically, the yield of heavy oil was 16.98% at 250℃; it was maximized at 19.57% when temperature was 275℃. At this point, the solid residue yield was decreased from 32.14% to 23.00%. When the temperature range was between 275℃and 325℃, the yields of heavy oil and solid residue did not change significantly. When residence time was less than 15 min, heavy oil yield was increased firstly and then decreased. This yield peaked at 24.14% when the residence time was 10 min. Meanwhile, the solid residue yield was decreased firstly and then increased. The yields of light oil and gas did not change considerably. When the residence time was extended to 30 min, the heavy oil yield was decreased to 10.86%. The solid residue yield was decreased firstly and then increased. Furthermore, the yield reached 28.67% when residence time was 25 min. Particle size had little influence on the hydrothermal conversion of cotton straw relative to the reaction temperature and time. The addition of catalysts increased heavy oil yield and reduced that of solid residue. The yields of light oil and gas did not change significantly. The influences of catalysts on heavy oil yield were ranked as follows: K2CO3>KOH>Na2CO3>Ca(OH)2>NaOH. When K2CO3 was used as the catalyst, heavy oil yield was increased from 18.00% to 29.86%. The results of analysis with gas chromatography-mass spectroscopy (GC-MS) indicated that the liquid product contained organic components, namely, acids, esters, aldehydes, ketones and phenols. Among these components, acids, esters, phenols and their derivatives were dominant. The addition of catalysts increased oil yield and also affected the oil components. Specifically, acids and ketones were reduced and the pH value of the oil was increased. As a result, its quality was improved to a certain extent. The results can provide a reference for biomass hydrothermal treatment.