A signal processing method and a signal processing system are provided to convert optical or electric signals by an effective circuit to have an increased dynamic contrast and reduced noises. The signal processing system includes an analog signal processing module and a digital signal processing module. When an optical signal of an object-to-be-detected is strong, an image-to-be-detected is obtained by an analog signal processing method. When the optical signal of the object-to-be-detected is weak, the image-to-be-detected is obtained by a digital signal processing method after background noises are filtered out.