This work aims at online calibration of signal attenuation of the long cable used in Tibet water Cherenkov muon detector array-A (Tibet MD-A) under the Tibet air shower array. Based on a waterproof connection of the signal cable to PMT (R3600_06) and characters of the high voltage divider, terminal reflection method is used for measuring the signal attenuation with a practical way to eliminate contribution of the pulse baseline. Comparison measurement data-taking method (with charge-to-digital conversion) was carried out by using open-ended cables, confirming that terminal reflection method is a fast and convenient, and suitable to online calibration of the signal attenuation for Tibet MD-A. At 26℃, the measured attenuation coefficient with the 250-m cable connected permanently to the PMT, was ~13.9%. The cable frequency response was measured by using the sinusoidal signals. The method could be used to study time dispersion of signals produced with Tibet MD-A detector via Fourier analysis.