The invention discloses a control method for an air-out volume of a vehicle-mounted air conditioner. The control method comprises detecting a current number of passengers in a rear row in a vehicle body, comparing the current number of the passengers with a previous number of the passengers, increasing the air volume of a rear-row air outlet of the vehicle-mounted air conditioner if the current number of the passengers is greater than the previous number of the passengers and decreasing the air volume of the rear-row air outlet of the vehicle-mounted air conditioner if the current number of the passengers is less than the previous number of the passengers. The control method for the air-out volume of the vehicle-mounted air conditioner disclosed by the invention can increase the air volume of the rear-row air outlet of the vehicle-mounted air conditioner if the current number of the passengers is greater than the previous number of the passengers and decrease the air volume of the rear-row air outlet otherwise, so that the air volume of the rear-row air outlet can be adjusted according to the variation of the number of the passengers in the rear row in the vehicle body so as to allow the air volume allocated to each passenger is well-distributed and appropriate, make the passengers be comfortable, avoid situations of supercooling or overheating and improve the riding experience. The invention also discloses a control system for the air-out volume of the vehicle-mounted air conditioner and a vehicle, all of which have the same advantages.