A projection device and an automatic projection calibration method thereof are disclosed. When the projection device boots up, an image pattern corresponding to a display data is projected through a projecting unit. The image pattern is sensed through an image sensor to obtain information of the image pattern. The information of the image pattern is compared with information of a previous image pattern through a processing unit to determine whether to execute an automatic calibration procedure. When the processing unit executes the automatic calibration procedure, a calibration pattern is projected through the projecting unit. The calibration pattern is sensed through the image sensor to obtain information of the calibration pattern. A calibration driving procedure is executed through the processing unit to convert imaging coordinates of the calibration pattern sensed through the image sensor into projecting coordinates of the projection device, so as to establish a coordinate conversion table.