The phase noise of quantum dot lasers is investigated theoretically by coupling the Langevin noise sources into the rate equations. The off-resonant populations in the excited state and in the carrier reservoir contribute to the phase noise of ground-state emission lasers through the phase-amplitude coupling effect. This effect arises from the optical-noise induced carrier fluctuations in the off-resonant states. In addition, the phase noise has low sensitivity to the carrier scattering rates.