Distributed feedback (DFB) semiconductor lasers have the advantages of small size, low cost and mature manufacturing technology. However, their application scope is limited due to the MHz linewidth. The linewidth can be narrowed to the order of kHz by selfinjection locking with ring resonators, but there exists instability as the laser being locked. We selfinjection lock a DFB semiconductor laser to four different ring resonators and monitor the changes of optical power, polarization state and wavelength at multiple ports of the locking circuit. It is revealed that the factors affecting the locking stability of DFB semiconductor laser include resonant mode hopping, polarization state hopping, and phase change of the feedback locking loop caused by external temperature and vibration, and the dominant factors are different when different types of ring resonators are used. Control of these factors can improve the stability of the selfinjection locking of DFB semiconductor lasers, so that better application effect can be achieved for the selfinjection locked DFB semiconductor laser.