The study was performed to evaluate the effects of thermal modification on the mechanical properties, optimize the modification temperature based on strength class, and provide a basis for the rational application of thermally modified wood in buildings. In this study, a total of 560 poplar wood specimens were tested to determine the effects of thermal modification between 160 and 210 ℃ on mechanical properties, such as bending strength (fm), parallel-to-grain tensile strength along the grain (ft, 0), perpendicular-to-grain tangential (ft, T, 90) and radial tensile strength (ft, R, 90), parallel-to-grain compressive strength (fc, 0), parallel-to-grain tangential (fv, T) and radial shear strength (fv, R), and modulus of elasticity (E0). The Fourier transform infrared spectroscopy was used to analyze the changes in chemical components of thermally modified wood at different temperature levels. The optimization temperature of thermal modification based on strength class was put forward. The results showed that the hemicellulose within wood had the lowest heat resistance under high-temperature condition and was first degraded by thermal exposure followed by acceleration at ≥190 ℃. The thermal resistance of cellulose was relatively higher, which was slightly degraded at the higher temperature, and mainly occurred in the amorphous region, increasing the orderly arrangement of microfibril. It was shown that thermal modification had an obvious adverse effects on fm, ft, 0, ft, 90 and fv of poplar wood. At room temperature, fm, ft, 0, ft, T, 90, ft, R, 90, fv, T　and fv, R were determined as 67.0, 86.2, 5.8, 8.9, 7.7 and 6.7 MPa, respectively. At lower temperatures, the chemical components of wood degraded slightly, and the mechanical properties of thermally modified poplar wood specimens decreased slowly. These parameters decreased to 53.5, 78.9, 4.0, 4.8, 6.0 and 5.4 MPa at 180 ℃, respectively. When the temperature was ≥190 ℃, severe pyrolysis occurred to the main chemical components, resulting in the rapid reduction of mechanical properties. At 210 ℃, these parameters represented 44.5%, 56.1%, 43.1%, 29.2%, 34.5% and 26.7% of the values at normal temperature, respectively. fc, 0 and E0 of thermally modified poplar wood increased as the temperature increased from 160 to 180 ℃, then decreased in the temperature range between 190 and 210 ℃. fc, 0 and E0 were 41.4 and 8 568 MPa at 20 ℃, respectively. In the temperature range between 160 and 180 ℃, the crystallization of cellulose increased, leading to the increase of the two parameters. At temperature of 180 ℃, fc, 0 and E0 were 30.7% and 12.8% higher than those at room temperature, respectively. The pyrolysis of cellulose increased with the temperature, resulting in the two values decreasing continuously until they achieved 45.0 and 8 104 MPa at 210 ℃, respectively. The untreated poplar wood cannot be used as a structural material, because E0 does not meet the requirements of the minimum strength class D18 according to European standard BS EN 338. The E0 of the modified wood specimen between 160 and 170 ℃ was higher than that of the untreated, but it was still lower than that specified by the minimum strength grade D18. After that, E0 increased with increasing temperature, and the modified poplar wood reached strength class D18 at 180 ℃. At temperatures between 190 and 200 ℃, the E0 of thermally modified wood specimens was higher than the corresponding value of strength class D18, but they still cannot be used as loading-bearing materials due to the excessive reduction of fv, R. The study can provide a basis for the rational application of thermal modification technology and low-quality fast-growing wood in engineering structures.