Local environment control system (LECS) has been widely used in many areas as the necessary guarantee for most precision instruments. A sealed air cooling constant temperature box with dual-chamber structure that can provide constant working condition for precision instruments in-vehicle was developed. According to the heat transfer analysis of the double plate-fins heatsink, the heat radiation area and dimensions were determined based on electrical analogy. The air flow rate and temperature distribution of the system in heat steady-state was simulated by CFD software, and the results are consistent with that of experimental test. In different conditions, the thermal test results indicate that when the heat flow rate in the inner chamber is about 170 W, the temperature difference between inner chamber and environment is no more than 6.5 °C. The thermal design and test methods can be as the reference for the design of other precision instruments in-vehicle.