Abstract: At 13:24:50 on July 18, 1969, an earthquake with magnitude of 7.4 (Mb) occurred in the Bohai sea, with its focal depth being 35 km. From the distribution of crustal thickness, it can be seen that the crustal thickness in the epicentral area is 29 km. So the Bohai earthquake is quite possibly a mantal earthquake. The deep structure of this earthquake has been discussed by analysis of seismic activity and macroscopic characteristics, combining with crustal thickness, the undulation of Curie's isotherm surface, the electrical structure of upper mantal of the crust and rock experiment material.