Abstract: Disclosed are a method for displaying projection picture and a method for rotating projection picture, comprising following steps: A: using a main control chip of a display to receive a picture signal parameter of a handheld terminal; B: the main control chip determining the black edge and calculating the starting position of a display picture by detecting the intensity of display signal inputted from the handheld terminal; C: obtaining the width of the display picture by calculation, and at the same time, directly obtaining the height of the display signal by signal detection of the main control chip; and D: using the function calculation provided by a control unit to enlarge the picture to a proportional or full-screen output with the handheld terminal. After adopting the above method, the display provides a picture with “full screen without having black edges” or “small black edges” to enhance the visual experience.