For the research on the fluid flow in the porous medium of graphite, the law of non-Darcy flow was revealed through the seepage flow of single-phase water in the saturated carbon-graphite with the whole Core Flow System.The experiment results indicate that the flow of water deviates from Darcy's law, the flow curve is nonlinear, and it can be described in a quadratic equation. The fitting precision is higher, and the correlation coefficient is more than 0.99. The flow has minimum starting pressure gradient in the seepage process and its value decreases rapidly with increasing porosity.Graphite materials can be divided into three categories according to the minimum starting pressure gradient with the change in porosity, namely “easy infiltration category”, “difficult seepage category” and “transition category”. Their porosity is greater than 18%, less than 11% and in 11%~18%. The nonlinear seepage law of water is verified through Reynolds number criterion and it agrees well with the experimental phenomena.