The work aims to study the thermal safety and optimization measures of used lithium battery transport packaging boxes for vehicles. With 15 groups of square lithium battery modules weighing 1.3 kWh as the study subjects, transport packing boxes were designed according to the P911 and LP906 standards. A model for used vehicle lithium battery transport packaging boxes was produced. After the model was experimentally validated, a temperature field simulation analysis was performed, and the liquid collecting tank, wall thickness, and thermal insulation material of packaging boxes were optimized. The issue of temperature above 100 °C could be resolved by adding flame-retardant cotton to the bottom of the liquid collecting tank. The optimal results were achieved with 15 mm flame-retardant cotton in the box and 40 mm flame-retardant cotton at the bottom of the liquid collecting tank. Cotton with nano-silica fiber could boost its heat-insulating performance by 16.0%. Finally, the entire packing box satisfied the standards of P911 and LP906, saving 68.75% of the flame-retardant cotton material and increasing the heat-insulating effect of the liquid collecting tank by 41.5%. The study's findings can be a valuable resource for future research on the packing of used lithium batteries for vehicles that are classified as hazardous products.