From the Φ-OTDR test environment built in the UHV DC control and protection engineering system, we analyze the characteristics and statistical law for fault detection signal in both the frequency and time domains along the optical cable link. The expressions of short-time over-level rate and addition short-time over dual-level rate for the signal in the time domain are given. Furthermore, we propose a synthesis algorithm of the time-domain multiplication short-time over dual-level rate and frequency domain-segmented band RSMD. Results show that the short-time dual-level rate is the best, and the SNR of the Φ-OTDR system can be improved to 9.41 dB, 9.02 dB, 7.60 dB, and 3.50 dB, under the fault positions of 2.3, 5.0, 10.8, and 16.0 km, respectively, along the cable link. The SNR of the Φ-OTDR system under the fault position of 2.3 km is further improved when N=8 and the peak value of the frequency band RSMD highlighted the weak fault position, which is a little different from the actual situation. Combining both algorithms can reduce the signal processing time and the weak fault location accuracy along the optical-fiber link to ±2 m. Additionally, the proposed algorithm improves the SNR of the Φ-OTDR optical cable link fault detection signal in the UHV DC control and protection engineering system, which identifies weak faults along the optical cable link and realizes the accurate positioning of weak fault events.