In order to solve the problem of non-uniform electric field distribution in converter transformer barrier system caused by conductivity difference between the transformer oil and pressboard, the SiC(silicon carbide) modified pressboard was developed by nano doping method. The conductivity of modified pressboard was measured. The measuring results show that the conductivity of modified pressboard increases exponentially with the increase of nano SiC doping ratio. The electric field strength has obvious influence on conductivity at high nano doping ratio, which shows great nonlinear characteristic. Based on the nonlinear conductivity characteristic of modified pressboard, the homogenizing effect of the nano modified pressboard on electric field distribution was verified by simulation. The simulation results show that the electric field distribution under DC and polarity reversal voltage in barrier system can be well homogenized by using the nonlinear characteristic of nano modified pressboard.