An automatic focusing projecting device includes an image light source, a first image lens, an image sensor, a second image lens, a memory, a microprocessor, and at least a driving device. The image light source projects an image onto a screen through the first image lens. The image sensor senses the image on the screen through the second image lens. The microprocessor calculates sensed signals from the image sensor to get a definition of the image, and then saves the definition in the memory. The microprocessor compares the definitions when the first image lens and the second image lens are moved to different positions to move the first image lens and the second image lens and locate the first image lens and the second image lens at their focusing positions respectively where the definitions of the image are the greatest.