The mathematical model of hydraulic of electro-hydraulic power steering system(EHPS) line was established by using bode graph theory and the state space equation was attained. Dynamic fluid friction of the line has been considered when modeling so the simulation results more agreed with reality. A simulation was carried out to analyze how the fluid resistance, induction and capacity impacted the work performance of the hydraulic line. The result is that, increase the fluid resistance and induce or decrease the fluid capacity could all boost up the stability.