A method and system for preliminary data processing before printing is disclosed. The steps executed by said system comprises: dividing a page into a plurality of bands including a Mth band and a (M+1)th band; converting a color format of the Mth band; transferring the Mth band to a printer for storing, and converting the color format of the (M+1)th band in the period of transferring the Mth band; and printing the Mth band, wherein the printing does not start until the printer receives 50%-100% bands. During the printing, the color format of the residual bands continues to convert and the converted residual bands continue to transfer to the printer until the page is finished printing.