Exploration of lignocellulosic biomass provides a sustainable and eco-friendly route for producing liquid fuels, materials, and chemicals. However, direct utilization of lignocelluloses is limited by the stable and complicated cross-linking structure of the plant cell wall. Hydrothermal pretreatment (HTP) is a green and cost-effective technology because it can disrupt lignin-carbohydrate complex (LCC) linkages, dissolve hemicelluloses and lignin, and redistribute lignin in the cell wall layers without utilization of any chemicals. Thus, HTP is expected to achieve industrial scale in second-generation biorefineries and circular bioeconomies. This review analyzed the deconstruction of lignocelluloses by HTP, with particular emphasis on the formation mechanism of hemicellulose degradation products and the structural evolution of hemicelluloses and lignin accompanying HTP. Meanwhile, the formation mechanism of pseudolignin and its effect on the enzymatic hydrolysis of cellulose as well as strategies for inhibiting lignin recondensation were discussed. •Reviewing the hydrothermal deconstruction of lignocelluloses.•Potential degradation products in hydrolysates are focused.•Formation of pseudolignin and its inhibition on enzymolysis are discussed.•Tracking the structural changes of hemicelluloses and lignin during the treatment.•Strategies to suppress the acid-catalyzed condensation of lignin are summarized.