As the control system without position sensors for the permanent magnet synchronous motor is of low con-trol accuracy at low or zero speed，a subsection control strategy with both the separately-controlled startup and the self-controlledoperation is proposed． In this strategy，when the system is in the separately-controlled startup stage，the machine can be started or stopped smoothly by injecting a stable rotating magnetic field into stator windings．However，when the system is in the self-controlled operation stage，a fractional-order sliding mode observer and a fractional-order model areboth introduced to ensure the smooth transition between the two control stages in the sen-sorlesscontrol system． Then，the response curves of the sensorlesscontrol system respectively with fractional-order sliding mode observerand integral-order sliding mode observer are obtained via simulation and experiments． The re-sults show that the proposed algorithm not only implements smooth startup of the machine but also effectively re-duces the chattering in the switching process． Thus，a higher integrated control performance is achieved．