An optical harmonic mixing system based on femtosecond laser technology is designed and built for ultrawideband microwave/millimeter wave measurement. In the system, femtosecond laser pulse train is utilized as the light carrier, and the signals under test are mixed with the high order harmonics of the femtosecond pulse envelope function, and then are down-converted into direct current (DC) for further measurement. Since femtosecond pulse train is characterized with wide-banded harmonics, the present system could cover a measurement bandwidth of 100 GHz. Aperture electric field amplitude distributions of 6.56 GHz and 100 GHz antennas are measured, respectively, using such system and the experiment results are corresponding well with electromagnetic simulations, which proves the correctness of the design. Besides, the harmonic mixer is achieved by using an electro-optical crystal(i.e. LiNbO3) based on Pockels′ effect.