An USB OTDR (Optical Time Domain Reflectometer) is disclosed to include an OTDR (Optical Time Domain Reflectometer) module adapted to test a fiber-optic in a fiber-optic transmission system, and an USB module adapted to connect the OTDR module to an external device to obtain power supply from the external device, for enabling the external device to control the OTDR module to test the linked fiber-optic and to collect feedback data.