The principle and configuration characteristics of fiber delay lines (FDL) are analyzed, which are becoming research hotspots. Categories of fiber delay lines are concluded and three important application fields of fiber delay lines are tailed after particularly. At first, in fiber sensing and optical measurement field, the paper analyzes laser line-width measurement system, crystalloid birefringence measurement system, optical coherence tomography (OCT) measurement system and optical correlation spectroscopy (OCS) measurement system. The primary function and parameter setting of fiber delay lines in these systems are pointed out. Secondly, it analyzes the principles and system configurations of high-speed pulse code generator, hybrid optical buffer and optical encoder /decoder in optical communication field, including some essential problems. Thirdly, in detail, it analyzes multi-tone photonic oscillator, microwave fiber delay line, optical A/D converter and optical signal correlation processor in microwave photonics field. At last the paper proposes that the research directions of fiber delay lines are improving delay time precision, realizing delay time regulation continuously, reducing insert loss and improving engineering reliability. Overcoming these problems is the key for wide application of fiber delay lines.