It is suggested that the capillary pressure-fluid saturation relationship should be determined as a function of a dynamic coefficient (tau) and the time derivative of fluid saturation (partial derivative S-w/partial derivative t), indicating a dynamic capillary pressure in most cases, which will increase the flowing resistance and injection pressure for oil-wet reservoirs, especially in low permeability formations. To decrease the injection pressure and improve injection, various chemical additives such as surfactants and fluorides have been widely used in the waterflooding process in low permeability reservoirs. Effects and mechanisms of these chemical additives are yet not well-known. In this paper, a series of specially designed waterflooding experiments were conducted to investigate the effects and mechanisms of surfactant additives on the dynamic capillary pressure-fluid saturation relationship in low permeability reservoirs. In the experiment, capillary pressure-fluid saturation relationships in three low permeability core samples were examined during the waterflooding process, as well as the surfactant added waterflooding process. Then, local dynamic coefficients in the core samples were calculated and compared. Results indicate that low permeability reservoirs present a high dynamic coefficient, therefore generating high dynamic capillary pressure, which is the cause of high injection pressure during waterflooding. Furthermore, surfactant additives can reduce the dynamic coefficient and capillary pressure significantly, and the lower permeability core sample shows higher dynamic capillary pressure reduction, indicating that surfactant added waterflooding can significantly reduce injection pressure in low permeability reservoirs. This work provides a method to investigate the interaction among fluids and porous media during waterflooding through the examination of dynamic capillarity.