Considering the needs of lowcost, high precision, and convenient in measuring crop leaf area, using a computer and a scanner, choosing different scan modes, by means of the measurement methods of with and without Baseplate, with and without PET FILM, and different shaped samples, it is concluded that the measurement methods of without baseplate and without PET FILM 300 dpi gray scale are better than others, with the measurement error is 0.4‰. Based on computer image processing technology, the software of measuring crop leaf area is developed by scanner, which achieves such functions as parameter selection, displaying, storing and invoking image and data, data processing, leaf area index calculation and so on.