A watermark generating circuit includes: a first computing circuit, for generating a second reference signal according to a computing parameter, an input data and a first reference signal; a second computing circuit, coupled to the first computing circuit, for generating at least one selecting signal to determine an embedding location for a watermark according to the second reference signal; and a register, coupled to the first computing circuit, for registering the second reference signal to transmit the registered second reference signal to the first computing circuit for updating the first reference signal, and for generating the watermark according to the second reference signal.