A spread spectrum clock generator includes a triangular wave generator, a digital wave modulator, a sigma delta modulator, and a selector. The triangular wave generator transforms one of the input clock signals into an original triangular wave signal, in which the input clock signals have the same frequency and phases different from each other. The digital wave modulator adjusts the waveform of the original triangular wave signal to generate an adjusted triangular wave signal and a first square wave signal according to an inputted control signal. The sigma delta modulator, electrically connected to the digital wave modulator, accumulates magnitude values of the adjusted triangular wave signal to generate a second square wave signal. The selector selects one of the input clock signals as an output clock signal based on voltage levels of the first square wave signal and the second square wave signal.