This paper presents a new method——Harmonic-Mixing Pulse Eleotro-Optio Probing for measureing the distribution of high frequency electric field: its principle, experimental apparatus, and its application to the measurement of standing-wave in GaAs coplanar waveguide. Experimental results gotten from measurements at frequencies up to 20.10 GHz and with different terminations (short,open and 50 ohms) are in good agreement with that predicted by theory. A discussion on the sensitivity and spatial resolution of tliis measurement mfethod is given.