The invention pertains to the technical field of synchronous motor control, and provides a synchronous motor start control method and system. According to the invention, after the synchronous motor is powered on, zero-current closed-loop control is performed on the synchronous motor at first, and the motor rotating speed is acquired at the same time to detect the initial rotating speed of the synchronous motor, and then when the motor rotating speed is greater than a preset rotating speed value (i.e., the initial rotating speed is greater than the preset rotating speed value), zero-current closed-loop control is performed on the synchronous motor continuously, and speed closed-loop control is performed on the synchronous motor according to the magnetic pole position and the rotating speed of the motor rotor so as to make the motor rotating speed smoothly changed to the object rotating speed to successfully complete the start process when the initial speed of the synchronous motor is greater than the preset rotating speed value, and no current abrupt change will occurs, no torque impact will be generated on loads, so the problem that the start process cannot be performed successfully with an existing synchronous motor which is not provided with a position sensor caused by the reason that the rotating speed of the motor before starting is not zero can be solved.