Abstract: The existing models for filtration rate modeling are based on the classic filtrateloss theory that filtrateloss velocity is inverse proportional to the exponent 1/2 of filtrateloss time, so they can’t be used to filtration rate modeling in naturally fractured formations. To take account of fracturing fluid leaking into the formation through natural fractures and the matrixes, a new model is set up in this paper for modeling fluid filtration rate in naturally-fractured and low-permeability formations and its exact solution is present by means of orthogonal transformation,the new model is easier to convergence and has a fast counting velocity.It is shown from study that the fluid filtration rate in naturally-fractured formations should not use the classic filtration rate theory