We have investigated experimentally the causes for degradation of the isolated stripe CW(GaAl)As/GaAs DH lasers by proton bombardment. The results of ageing tests show that the fast degraded lasers are caused by defects (such as dark spot, dark line and dark region) in the active region extendeing rapidly during the test process. One of the factors limiting the lifetime to 103 hours is the oxidization of the mirror facet of laser cavity; and the moving of mierodefects by proton bombardment into the active region becomes one of the reasons limiting the lifetime to 104 hours.