This paper presents a theoretical investigation of the phase jump of multiphoton harmonic emission driven by two frequency-comb fields. The Floquet theorem is employed to provide a nonperturbative and exact treatment of the interaction between a quantum system and frequency-comb fields. Multiple multiphoton-transition paths for the harmonic emission are coherently summed. The phase information about paths can be extracted via the Fourier Transform analysis of the harmonic signals which oscillate as a function of the relative phase between two frequency-comb fields. Phase jumps were observed when harmonic emission was sweeping across the resonance by varying the frequency or intensity of two frequency-comb fields, which allows us to observe the Stark-shifted transition energy of resonant frequency of quantum system driven by strong laser fields.