A heat dissipation system in a box type chassis of a communication device includes an upper heat dissipation channel and a lower heat dissipation channel, which both have an air exhaust vent and an air intake vent, the air intake vent and the air exhaust vent of the upper heat dissipation channel and the lower heat dissipation channel are defined on sidewalls of the box type chassis, and the air intake vent of the upper heat dissipation channel is located above the air exhaust vent of the lower heat dissipation channel. An air baffle is arranged in the box type chassis to generate a circulative airflow in the box type chassis, and therefore, heat dissipated by a circuit board is circulated in the box type chassis through the airflow, the temperature in the box type chassis reaches an extent that prevents vapor from condensing or even becoming a water film.