A signal processing system includes a module under test, an oscillation signal generator, a translational filter, and a testing module. The module under test has a signal input end. The oscillation signal generator generates an oscillation signal. The translational filter includes a mixer controlled by the oscillation signals. The mixer has a high-frequency side and a low-frequency side. The high-frequency side is coupled to the signal input end of the module under test. The testing module is coupled to the low-frequency side of the mixer. When the signal processing system is in a testing mode, the testing module provides a testing signal to the low-frequency side, so as to generate a high-frequency testing signal at the high-frequency side of the mixer.