A passive microreaction system with excellent mixing performance was used for the optimization of the size and shape of silver azide (SA) at the microscale. This safe and cost-effective method is characterized by high mixing efficiency, low reagent consumption, and rapid preparation. To exploit the difference, the preparation of SA at the microscale using the passive microreaction system was compared with that at the macroscale in a beaker. The results show that the preparation of SA using the passive microreaction system under microscale conditions has obvious advantages over its preparation in a beaker in terms of crystal morphology, particle size, particle size distribution and thermal stability. The shape of SA prepared in the system is spherical or spherical-like, while the morphology of SA synthesized in a beaker is mostly pyramidal with sharp points. The particle size of SA prepared in the microreaction system ranges from 712.4 nm to 1106.4 nm, while that of SA prepared in a beaker ranges from 255.0 nm to 825.0 nm. In addition, the detonation velocity of submicron-SA is 1850 m s⁻¹, which is 150 m s⁻¹ higher than that of confined SA published in the literature. This study demonstrates the feasibility of a safe and efficient method for fast preparation of SA with improved physical properties.