The invention discloses a flux weakening control method for a permanent magnet synchronous motor, a flux weakening control device, a flux weakening control system and a vehicle. The method comprises steps: a first current value and a second current value of the permanent magnet synchronous motor are acquired; the first current value is adjusted through a first current reference value to obtain a first adjustment current value, and the second current value is adjusted through a second current reference value to obtain a second adjustment current value; vector control is executed on the first adjustment current value and the second adjustment current value to obtain a first voltage value and a second voltage value; calculation is executed on the first voltage value and the second voltage value, a PWM modulation degree is obtained, flux weakening feedback is executed according to the PWM modulation degree, and a flux weakening current feedback value is obtained, wherein the flux weakening current feedback value is used for adjusting the first current reference value; and the PWM modulation degree is adjusted to the best modulation degree through the flux weakening current feedback value, wherein in the best modulation degree, the permanent magnet synchronous motor operates in the flux weakening operation region. Thus, the permanent magnet synchronous motor operation stability can be improved.