Taiwan Strait is located between the actively deforming Taiwan Island and stable South China Craton. While strong earthquakes have occurred on the east and west sides of the Taiwan Strait and their seismotectonic are relatively well understood, the earthquake risk and seismogenic structure in the strait still need more investigations. On November 25, 2018, a rare moderately strong earthquake with a magnitude of Mw5.8 occurred inside the strait, and it caused broad ground motions in the near regions. From the waveform inversion of the seismic data, this earthquake is found to be a high-dip angle strike-slip event, with the two nodal planes striking east-west (EW) and north-south (NS) directions. As there are mapped faults striking EW and NS in the epicentral region, further studies are needed to determine the ruptured fault plane. To determine the ruptured fault plane, we analyzed the rupture directivity of the mainshock with the relative location between the hypocenter and the centroid. We chose several M4+ aftershocks as reference events and relocated the reference earthquakes with the Pn travel time difference. Then, we used regional waveform data to invert centroid location and source parameters of earthquakes. Thereafter, we estimated the strike of the mainshock ruptured fault and rupture length, via fitting the azimuthal variation of the Love wave time shift difference between the mainshock and the reference earthquakes. Our results showed that the mainshock ruptured along NS strike with the rupture length about 3.6–5.6 km. Combined with the tectonics, we inferred that the seismogenic fault is the Dongshan uplift eastern fault (DSEF) and the 1994 event may promote the occurrence of the 2018 Mw5.8 earthquake.