An optical time-domain reflectometer OTDR, comprising a controller, a laser emitting apparatus, a signal modulation apparatus, a coupler, a signal coupling apparatus, and a receiver, wherein the laser emitting apparatus is configured to output a first optical signal in a first time period based on a first control signal sent by the controller; the coupler is configured to couple the first optical signal to the receiver and the signal modulation apparatus; the signal modulation apparatus is configured to: generate a pulse signal based on the first optical signal, and output the pulse signal in a second time period to an optical fiber based on a second control signal sent by the controller, wherein the first time period comprises the second time period; the signal coupling apparatus is configured to: couple the pulse signal to an optical fiber, and couple a scattered signal from the optical fiber to the receiver, wherein a frequency of the scattered signal is the same as a frequency of the first optical signal; the laser emitting apparatus is configured to output a second optical signal in a third time period based on a third control signal sent by the controller, wherein a frequency of the second optical signal is different from the frequency of the first optical signal, and a start moment of the third time period is later than a stop moment of the first time period; the coupler is configured to couple the second optical signal to the receiver; and the receiver is configured to perform coherent detection based on the second optical signal and the scattered signal from the optical fiber, wherein the second optical signal differs in frequency with regard to the scattered signal, and the second time period starts after the first time period has ended. the laser emitting apparatus is configured to output a first optical signal in a first time period based on a first control signal sent by the controller; the coupler is configured to couple the first optical signal to the receiver and the signal modulation apparatus; the signal modulation apparatus is configured to: generate a pulse signal based on the first optical signal, and output the pulse signal in a second time period to an optical fiber based on a second control signal sent by the controller, wherein the first time period comprises the second time period; the signal coupling apparatus is configured to: couple the pulse signal to an optical fiber, and couple a scattered signal from the optical fiber to the receiver, wherein a frequency of the scattered signal is the same as a frequency of the first optical signal; the laser emitting apparatus is configured to output a second optical signal in a third time period based on a third control signal sent by the controller, wherein a frequency of the second optical signal is different from the frequency of the first optical signal, and a start moment of the third time period is later than a stop moment of the first time period; the coupler is configured to couple the second optical signal to the receiver; and the receiver is configured to perform coherent detection based on the second optical signal and the scattered signal from the optical fiber, wherein the second optical signal differs in frequency with regard to the scattered signal, and the second time period starts after the first time period has ended.