The invention belongs to the field of energetic materials, and particularly discloses a nanometer explosive preparation system, based on passive micromixer, and a method. The system comprises a fluiddriving unit, a recrystallization unit and a sample collection unit which are arranged in sequence, wherein the recrystallization unit comprises the passive micromixer and a temperature control device; the fluid driving unit provides flowing driving force for solvents and nonsolvents; the passive micromixer realizes mixing of the solvents and nonsolvents through a micro-mixing structure; the temperature control unit is used for controlling the temperature during a nanometer explosive preparation process; the sample collection unit is used for preparing explosive suspension. The mixing strengthof the passive micromixer can be controlled through the adjustment of the structure and length of a micro-mixing channel, and can be applicable to more size controllable equipment for explosives. Asthe residence time of the solvents and nonsolvents in the recrystallization unit is short, size distribution of prepared nanometer explosive is narrow, the crystal morphology of the nanometer explosive is uniform, and the system has excellent physical and chemical properties and explosive property.