A method for converting a document into a full-page image using a graphical device interface (GDI) of an operating system is disclosed. The GDI provides objects of the document and coordinates of the objects in the document. The document is divided into a plurality of blocks. An address array having a plurality of address units is established to record the coordinates of the objects. Each of the address units corresponds to one of the blocks and is used to record positions of the objects in the block. The positions recorded in the address units are updated according to the coordinates of the objects provided by the GDI. A corresponding image piece for each block is generated according to the updated positions recorded in the corresponding address unit. All of the image pieces are merged into the full-page image.