An image noise reduction method based on an optimal primary-color signal is suitable for a digital camera to reduce noises in a digital image. The image noise reduction method mainly includes the following steps, taking out primary-color signals of a digital image, including taking out high-frequency and low-frequency signals of the optimal primary-color signal and high-frequency and low-frequency signals of the worst primary-color signal by using a high-pass filter and a low-pass filter; next, mixing the high-frequency signal of the optimal primary-color signal with that of the worst primary-color signal, and superposing the mixed signal to the low-frequency signal of the worst primary-color signal, so as to get a digital image signal after noise reduction.