This study aims to clarify the physicochemical properties and enzymatic digestibility of Steam Exploded (SE) maize stalk under the conditions of steaming temperature-explosion pressure decoupling during bioethanol production. A SE pretreatment was conducted at low-temperature maintenance, followed by pressure lifting and decompression, in order to realize the decoupling of steaming temperature and explosion pressure. Three treatments were performed to determine the hydrothermal effects of SE on the chemical properties of maize stalk at the cooking temperatures (453, 471, and 485 K), and the explosion pressure of 2.0 MPa. Another three treatments were also performed to explore the physical explosion effects of SE on the physical properties of maize stalk at the explosion pressures (1.0, 1.5, and 2.0 MPa), and the cooking temperature 485 K. The retention time was fixed at 10 min in all six treatments. A systematic characterization was then conducted to analyze the physical structure (including microscopic morphology and porous properties), chemical composition (including components content, functional groups, and degree of crystallinity), and thermodynamic properties of SE maize stalk. The results showed that the contents of hemicellulose and acetyl decreased by 50.69% and 67.11%, respectively, whereas, the content of lignin increased by 17.66%, in the solid fraction of SE maize stalk with the increment of steaming temperature. In the liquid fraction, the content of carbohydrates decreased by 45.37%, while the contents of organic acids and furfurals increased by 37.66% and 73.21%, respectively. The thermogravimetric analysis showed that the higher the steaming temperature was, the greater the peak height and area of pyrolysis III stage were. It infers that the degradation rate and weight loss rate of SE maize stalk were greater than the original, indicating better thermal stability. The porous properties of SE maize stalk were also improved to different extents, with the increment of explosion pressure. Specifically, the cumulative pore volume and pore area increased by 15.56% and 34.67%, respectively, where the average pore size was lifted from 1 534 to 2 073 nm, and the porosity was raised from 77.62% to 80.47%, while the permeability and tortuosity increased by 20.69% and 6.61%, respectively. In addition, the SE presented outstanding tearing effects on the fiber bundles, due to the increased number of cracks in fibers, the raised roughness of the structural surface, and the disordered arrangement of fiber bundles. As such, the hydrothermal modification was greatly contributed to the thermal degradation of structural components and enzymatic digestibility during the SE process of maize stalk, compared with the physical explosion. However, there was the reduced cellulose recovery that resulted from the excessive degradation of structural components under higher steaming temperatures. The cellulose hydrolysis rate of SE maize stalk reached the highest value of 87.99%, the cellulose concentration of enzymatic hydrolysate was 16.74 g/L, and the total glucose yield (including cellulose recovery and hydrolysis yield) reached up to 86.53%, under the SE condition of steaming temperature 471 K and explosion pressure 2.0 MPa. Therefore, the SE pretreatment strategy under low steaming temperatures and high explosion pressures can be beneficial to improve the conversion efficiency and product yield. This finding can provide a theoretical basis to reveal the physicochemical mechanism of SE pretreatment on lignocellulosic materials for the high efficient bioconversion.