Based on the mechanism of electro-hydraulic impulse, the plasm expanding model is established, and by applying principle of energy conservation, the mathematic model of the pressure of electro-hydraulic impulse chamber is built. The research shows that the pressure of discharge channels is inversely proportional to the expanding radius of plasm and to the square of discharge time, and it has nonlinear relation with breakdown voltage and capacitance. The calculated value is similar to the test result of References 6. The mathematic model provides theory basis for the application of electro-hydraulic impulse in engineering.