The invention discloses an extreme value method for downstream tunnel length for a closed ventilation system of an extra-long highway tunnel. The method comprises the following steps: 1) flue dust flow of a purified circulating airflow is calculated, 2) flue dust concentration is calculated when the purified circulating airflow and a parallel airflow of a short tunnel duct are converged, 3) flue dust flow of a newly added downstream airflow is calculated, 4) flue dust concentration of the downstream airflow is calculated, and 5) an extreme value of downstream tunnel length is calculated; a formula for the extreme value of the downstream tunnel length indicates that when the length of a downstream tunnel is equal to or greater than the extreme value of the downstream tunnel length, the fluedust concentration of the downstream airflow exceeds allowable concentration, and vehicles in the downstream tunnel are unsafe. Via the method disclosed in the invention, the extreme value of the length of the closed type controllable circulating ventilation downstream tunnel of the extra-long highway tunnel can be rapidly and conveniently determined; safety of the vehicles running in the downstream tunnel can be ensured.