A method for controlling a vehicular air conditioning system including: setting up a target indoor temperature of a vehicle determined by a user; inputting inside and outside temperatures of the vehicle and solar radiation with reference to a sensor mounted on the vehicle; calculating a target discharge temperature of a vent (t1); inputting the maximum evaporator temperature (t2); setting up a target evaporator temperature by comparing t1 and t2; calculating the opening degree of a temperature control door; measuring actual evaporator temperature while controlling the discharge capacity of a compressor; calculating the opening degree of the temperature control door by using the actual opening degree of the compressor discharge capacity control valve and the actual evaporator temperature.