The stability properties of semiconductor lasers strongly coupled to an externalcavity are theoretically analyzed.A strong feedback factor approximation is introduced tothe rate equations of external cavity semiconductor lasers.Through a gain-rate-frequencyellipe drawing,we get a result that there exists a theoretical limit condition on the stabilityof given semiconductor lasers with a strongly coupled external cavity. Technical methodscan not surpass this limit condition after the parameters of laser are given. We also concluded that stronger feedback and shorter length of the external cavity are beneficial to improving the stability of lasers.