An optical time domain reflectometer and a method for detecting an optical fiber are disclosed. In an embodiment the optical time domain reflectometer includes: a service signal generator, a signal coupler connected to the service signal generator, an electro-optical modulator connected to the signal coupler, a transmitter connected to the electro-optical modulator, a photoelectric detector connected to the transmitter, and an analog to digital converter connected to the photoelectric detector. The optical time domain reflectometer further includes: a digital signal processor, connected to the service signal generator, the signal coupler, and the analog to digital converter, and configured to generate compensation data and a PN code sequence, receive a sampled signal sent by the analog to digital converter, and calculate a first optical fiber function.