The pulse-to-pulse rms timing jitter of a 5.25-GHz quantum-dot (QD) two-section passively mode-locked laser is characterized through an all-microwave technique. The experimental phase noise spectra at different harmonics are in good agreement with previous diffusion-based theory. This theory is validated for a QD mode-locked laser device for the first time. This measurement technique provides a simple way to characterize the noise performance of a passively mode-locked laser. Furthermore, the average pulse-to-pulse rms timing jitter reduces from 295 to 32 fs/cycle via external optical.