Abstract: Lead-bismuth cooled reactors utilizing liquid lead-bismuth as a coolant in the primary loop system are an advanced nuclear energy system with significant developmental prospects. Benefiting from the excellent neutronic properties, superior heat transfer performance, remarkable γ shielding, and containment capabilities for radioactive products of lead-bismuth materials, these reactors can achieve higher power densities and a simplified system design, making it feasible to develop small capacity, ultra-long-life micro reactors. Micro lead-bismuth cooled reactors have unique advantages in the comprehensive utilization of nuclear energy and are a crucial option for future mobile energy supply technologies. Their miniaturization and lightweight design are key to enhancing the overall performance of lead-bismuth nuclear power systems. This paper focused on the challenges of multi-physics, multi-variable, and multi-constraint coupling effects in the design optimization of miniaturization and lightweight of micro lead-bismuth cooled reactors. Initially, an analysis of the impact of fuel/coolant and solid moderator/reflector layer materials on the core’s critical dimensions and mass was conducted to guide fuel/material selection. Following this, the reactor core’s multi-factor synergistic optimization design was carried out using the self-developed DOPPLER platform for multi-physics intelligent design optimization of lead-bismuth cooled reactors. This led to the proposal of a miniaturized and lightweight 5 MWt micro lead-bismuth cooled reactor conceptual design scheme, MILLER-5, which also involved an assessment of the reactor core’s physical characteristics and steady-state thermal-hydraulic properties. MILLER-5 is loaded with 139.8 kg of PuN-ThN fuel, uses ²⁰⁸Pb-Bi as the coolant, and solid ²⁰⁸Pb as the reflector layer material. The fuel assemblies are surrounded by the solid moderator ZrH_1.6 to enhance the initial k_eff of the core, thereby reducing the fuel load and core active zone volume. The core’s volume power density is 114.8 W/cm³, with a refueling cycle of 1 000 d. The reactor core features stable reactivity swing and a flat power distribution, with all reactivity coefficients being negative, and a substantial margin of steady-state thermal-hydraulic safety.