Indirect modulation with electro-optical crystal is a useful way to generate optical signal for underwater blue-green laser communication. However, as crystal surface is not strictly flat in practical application, light intensity distribution in the cross section is non-uniform, which would affect extinction ratio of modulated signal and system performance. In this letter, we study this issue with Monte Carlo method. The result shows that with the increase of crystal flatness, extinction ratio is decreasing dramatically, and it should be smaller than 0.78μm in order to make the extinction ratio greater than 10dB while 0.25μm for 20dB, 0.08μm for 30dB, and 0.025μm for 40dB. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.