In order to suppress the current harmonics of dual-three phase permanent magnet synchronous machines (DTP-PMSMs), a synchronous filtering (SF) based suppression is proposed in this paper. It is based on the extraction of the 5th, 7th, 11th and 13th current harmonics from $d_{1}-q_{1}$ and $d_{2}-q_{2}$ planes by synchronous filtering, which will be online minimized by a set of cascaded PI controllers afterwards. Its effectiveness is verified on a prototype DTP-PMSM and shows good performance in suppression of current harmonics.