A frequency up-conversion device and a signal transmission system are provided. A frequency up-conversion device includes a first diplexer, a divider, a digital channel stacking circuit, an up-conversion mixer, and a second diplexer. The first diplexer divides a first signal into a second signal and a third signal. The digital channel stacking circuit transforms the second signal to a fourth signal. The up-conversion mixer mixes the fourth signal and an up-conversion oscillating signal to generate a fifth signal. The second diplexer receives the fifth signal and the third signal to generate a sixth signal for output.