In the present work, the three‐dimensional thermal analysis of the process of the catastrophic mirror damage in double‐heterostructure stripe‐geometry GaAs/(AlGa)As diode lasers has been carried out for the case of their high‐power operation. In the analysis, the temperature dependencies of both the thermal conductivity and the thermal diffusivity have been taken into account. The analytical solution has been obtained with the aid of the Green’s function method. The catastrophic mirror damage has been explained from the thermal point of view as a result of a local overheating of the active area close to or at the mirror surface. It has been proved that a determination of an exact value of the critical mirror temperature from the catastrophic degradation point of view is of minor importance because the considered temperature increase is of an avalanche type. The analytical solution obtained in the work has enabled us to show a dependence of a catastrophic‐degradation time, i.e., a permissible length of current pulses from a point of view of the catastrophic mirror damage, on the amplitude of the pulses for the standard stripe diode lasers. Experimentally observed catastrophic mirror damage in diode lasers caused by an increase in their facet reflectivity has been also investigated. It has been confirmed that an increase in the reflectivity of an unprotected mirror may be accompanied by a very rapid facet failure. It has been also shown in which way one can avoid the above phenomenon.