A spread spectrum clock signal generator for spreading an input clock signal into an output clock signal includes a clock signal delay chain for delaying the input clock signal into a delay clock signal group having a plurality of delay clock signals, a modulation controller for outputting a counter clock signal control signal, a clock signal selection circuit for selecting, from the delay clock signal group, a modulation clock signal group having a plurality of modulation clock signals, a programmable counter for generating a counting value according to a counter clock signal, and a clock signal output unit for combining the modulation clock signals into the output clock signal according to the counting value, and further generating the counter clock signal, outputted to the programmable counter, according to the counter clock signal control signal.