Based on a two-dimensional axisymmetric thermal and mechanical simplified model of a quantum well infrared photodetector under 8.2 μm long-wave infrared mono-pulse laser irradiation, the transient temperature and stress-field distribution of the detector under various pulse widths were calculated using the finite-element method. In addition, variations in the thresholds of the detector for thermal decomposition, melting, and stress-induced damages under laser irradiation with the pulse width ranging from 1 to 300 ns were preliminarily determined. Results show that damage thresholds exhibit a downward trend as the laser pulse width decreases. Specifically, when the pulse width reduced from 300 to 1 ns, the decomposition damage threshold decreases by 23.10% (from 3.16 to 2.43 J/cm²), melting damage threshold decreases by 23.27% (from 5.50 to 4.22 J/cm²), and stress-induced damage threshold decreases by 44.62% (from 8.92 to 4.94 J/cm²).