We carefully reanalyze the ∼6-yr oscillation (SYO), which has been previously found in the Earth's length of day variation (ΔLOD) and has been considered important for understanding the core–mantle interactions and core dynamics. Based on the Morlet wavelet and AR-z spectra of a long-period yearly ΔLOD record (1760–2018), we identify eight long period signals, ∼149 yr, ∼68 yr, ∼33 yr, ∼22.3 yr, ∼18.6 yr, ∼13.5 yr, ∼11 yr and ∼8.5 yr signals; except the 18.6 yr signal, the other signals are clearly identified for the first time, although the physical mechanisms need to be understood (some of them may relative to the torsional waves or Magnetic-Archimede-Coriolis waves). After removing the atmospheric/oceanic angular momentum effects, the seasonal/tidal signals and the eight low-frequency signals from the 1962–2016 ΔLOD time series (from EOPC04), a much cleaner sequence for the SYO is obtained, and we confirm that there is no clear decay trend for the SYO in the residual sequence. After using a deconvolution method to this SYO time series, we first validate that the quality factor Q of the SYO cannot be less than 200. Assuming the attenuation of SYO is mainly caused by the electromagnetic coupling at CMB, and taking the mantle conductance Gm=108S, the radial magnetic field strength Bm at the CMB should be larger than 0.31 mT. In addition, for the first time, we calculate the excitation function of the SYO from the deconvolution of ΔLOD, and the results demonstrate that the SYO seems to be continuously excited. By comparing the geomagnetic jerks with the excitation function of the SYO, no clear correlation has been found between them, while we also cannot rule out the possibility of the geomagnetic jerks as one kind of the excitation sources of the SYO. Our findings are necessary for building a systematic physical mechanism of the SYO, and useful for constraining the magnetic field B of the Earth's core.