1. A watermark coding method for hiding a digital watermark into a digital image to generate a robust protection, applied in a recording media, the method comprising the steps of: determining a first pseudo random code PN 1 and using this PN code to perform a watermark series, Wj, conversion to generate a permuted watermark series Wi′; converting an original image Vm to generate a frequency domain of a modified original image Vm″, comprising the steps of; interleaving the original image Vm to generate an interleaved original image Vm′; and transforming the interleaved original image Vm′ into the frequency domain of the modified original image Vm″; using the frequency domain of the modified original image Vm″ and the permuted watermark series Wi′ to generate an encrypted watermark series Wi″, further comprises the steps of: determining a second pseudo random code PN 2 , a third pseudo random code PN 3 , and a threshold T; and using a frequency hopping rule to choose a subband coefficient B of the frequency domain of the modified original image Vm″ and determining an encryption means according to the relation between the subband coefficient B and the threshold T, thereby generating the encrypted watermark series Wi″; modulating the encrypted watermark series Wi″ to generate a modulated watermark series Wi′″; hiding the modulated watermark series Wi′″ in the frequency of the modified original image Vm″ by the frequency hopping rule; and converting the frequency domain of the modified original image back to its spatial domain to obtain the watermarked image Vm′″.