Optical gate based on electro-absorption modulator (EAM) now plays an important role in all-optical 3R regeneration, multiplexing, logic gates, and wavelength conversion for its polarization insensitivity and weak pattern effect. A novel numerical model on cross-absorption modulation (XAM) in bulk EAM is proposed. Using the finite-difference time-domain method, the optical gate in EAM is numerically calculated. And the influence of bias voltage, power and width of the pump pulse on the EAM gate is analyzed respectively. Related experiments concerning XAM is also conducted. Experimental results are in agreement with numerical calculation. Error-free 3R regeneration by an EAM as the reshaping optical gate is also performed for 40 Gb/s RZ signal that is degraded by residual dispersion or polarization mode dispersion.