The phase noise-induced interference (PNII) in coherently detected OTDR systems is investigated. A close-form relationship between signal to (interference) noise ratio (SNR) and laser linewidth is derived for the first time, to the best of our knowledge, and numerical simulations are conducted to verify the theoretical results. Additionally, the proportion of noise composition of PNII is studied. It is shown that the amplitude noise accounts for one-third of the total interference. This analytical form of PNII will assist in understanding the COTDR system that utilizes the full field of information (rather than intensity alone) at the receiver and, more importantly, provides a crucial guideline for designing high-performance and cost-effective COTDR systems in various applications.