The invention discloses a heat dissipating device, a back-light module and a displaying device of back light module. The heat dissipating device includes a heat sink used to install light source, on one side of which a heat sink shroud is formed. Each side of the heat sink shroud is provided with a gas inlet and a gas outlet. The outlet of a constant-flow fan is in correspondence with the gas inlet of the heat sink shroud. The backlight dissipation structure of the back-light module adopts a cross flow fan capable of guiding airflow into the heat sink shroud to remove heat and achieve the aimof active cooling.