A storage array module and a hard disk accommodating unit thereof are provided. The hard disk accommodating unit includes a fixing recess, a first tray and a second tray. The first tray is reciprocally and slidably disposed in the fixing recess. The second tray is reciprocally and slidably disposed on the first tray. When the second tray slides a distance in a sliding direction, the second tray pushes the first tray into the fixing recess, after the second tray reversely slides the distance in the sliding direction, the second tray pushes the first tray for protruding out of the fixing recess.