Symmetrical six-phase series-connected three-phase PMSM driven by single inverter is a novel multi-machines system. To research EMF harmonics influence on the decouple control of series-connected system, the series-connected system mathematic models with 2nd harmonic of symmetrical six-phase PMSM under d-q coordinates are deduced. The torque ripple amplitude and frequency influenced by harmonic are discussed. The decouple control strategy based on harmonic effect compensating is presented. And the simulations with variable operating states are proposed to validate the feasibility.