In order to solve the problem of loss measurement of ultra-long span fiber transmission, this paper proposes a new measurement method based on digital linear frequency modulation optical time domain reflection(DLFM-OTDR). This method can solve the contradiction between the maximum measurement distance and the spatial resolution. The DLFM-OTDR uses direct modulation and direct detection at the transmitter and receiver respectively. The structure is simple and requires no additional optics. The method introduces the short-term fractional Fourier transform (STFrFT) for signal processing and noise filtering. the paper uses the DLFM-OTDR development board to perform experimental tests on the actual fiber transmission links. The measurement results of the fiber link length of 2×150 km are taken as an example, the experimental results show that the DLFM-OTDR is especially suitable for monitoring the ultra-long fiber link under the condition of long single span.