The invention discloses a wind-dividing ratio maximum value determining method for the closed circulation ventilation of a super-long highway tunnel. The method comprises the following steps: firstly,the non-linear relation between the critical effective air volume of a dust collector and the partial wind-dividing ratio is determined; the first-order partial derivative and second-order partial derivative of the critical effective air volume of the dust collector to the wind-diving ratio are obtained by applying the derivation rule in higher mathematics. According to the determination method of extreme value, first-order partial derivative and second-order partial derivative in higher mathematics, when the first-order partial derivative is equal to 0, and the second-order partial derivative is less than 0, a maximum value exists, thus a formula for calculating the maximum value of the wind-dividing ratio is established. When the actual wind dividing ratio of the closed controllable circulation ventilation system is greater than the maximum value of the wind dividing ratio, the parallel air flow dust concentration in the tunnel short path exceeds the allowable concentration, the vehicles continuously running in the tunnel are not safe. The pre-evaluation of closed circulation ventilation system is quickly completed.