A new theory of wide bandwidth optical harmonic generation based on frequency mixing is presented.We point out that the wide bandwidth optical harmonic generation is a process of frequency mixing rather than only frequency doubling if there are phase match among different wavelengths.The sum frequency between different wavelengths can play a leading role in wide bandwidth optical harmonic generation,which can increase the acceptance bandwidth dramatically.This theory is not limited by the acceptance bandwidth of frequency doubling and not only suit for second harmonic generation but also for third harmonic generation.