The advantages of optical time-domain reflectometer (OTDR) using spread spectrum technology are analyzed briefly. For the shortcomings of spread spectrum OTDR in traditional despreading method, the concept of optical domain multiplier is first proposed, i.e., the most important multiplication operation in despreading is realized by modulating the phase of Mach–Zehnder modulator before photoelectric conversion. The proposed two methods of despreading in optical domain overcome the electronic bottleneck of correlative despreading in electrical domain, and solve the various restrictions on the application of high-speed spreading code in OTDR. Simulations verify the correctness and feasibility of the two methods.