The invention relates to a mandatory diffuse type cooling ventilating pipeline system for a large-scale frozen earth roadbed and a construction method thereof. The traditional roadbed ventilating pipeline system is limited in temperature reduction effect and is adverse to the stability of frozen earth by increasing the temperature of a roadbed by introducing hot air in summer. The mandatory diffuse type cooling ventilating pipeline system comprises a plurality of horizontal ventilating pipes and a plurality of longitudinal ventilating pipes, which are horizontally and longitudinally crossed and mutually communicated, wherein the longitudinal ventilating pipes positioned on the end parts of two sides are respectively connected with an air inlet and air outlets, which protrude out of the horizontal direction of the roadbed; one air inlet is formed, and an air suction equipment and a temperature controlled switch are arranged on the air inlet; and a plurality of the air outlets are form in the front and back sides along a longitudinal direction. According to the mandatory diffuse type cooling ventilating pipeline system, cold air is introduced from the air suction equipment arranged in the air inlet, circulated through the longitudinal ventilating pipes and the horizontal ventilating pipes and then emitted from the air outlets, and the air is subjected to heat exchange with the roadbed in a pipe internal-circulation process, so that the effect of reducing the temperature of the roadbed is achieved. The mandatory diffuse type cooling ventilating pipeline system disclosed by the invention can only be started when external air temperature is lower than the temperature of the roadbed to prevent the entrance of the hot air in summer and is controllable in air quantity and obvious in cooling effect.