The invention discloses image down sampling method and system, wherein the image down sampling method comprises the following steps: determining an image dissection layer level according to a predetermined image down sampling rate; carrying out a wavelet dissection on each component of an input image according to the determined image dissection layer level; determining a normalizing factor used for the normalization of lowest-frequency wavelet coefficients of all components after the wavelet dissection of the input image according to the lowest-frequency wavelet coefficient of each component after the wave dissection; normalizing the lowest-frequency wavelet coefficients of all components after the wavelet dissection to a time domain according to the determined normalized factor. The lowest-frequency wavelet coefficient of each component after the wavelet decomposition can be transformed to the time domain directly through the invention, thereby reducing the computation burden of the image down sampling method and system, improving transformation speed and saving computation resources.