Based on the composite cavity model, the feedback-induced instabilities which may cause coherence collapse in single-longitudinal-mode distributed feedback (DFB) fiber lasers are analyzed. The coherence collapse of DFB fiber laser is detected, and the longitudinal operating mode splitting phenomena of DFB fiber laser due to Rayleigh backscattering and the facet reflection are measured simultaneously. Experiments show that the threshold of the length of the leading fiber cannot exceed 200 m, which keeps quantitative agreement with the results of simulation. The investigation proves the feasibility of the two methods for detecting coherence collapse, i.e., mode splitting method and phase noise method, and provides a guide to the design of DFB fiber laser sensor array.