This paper utilizes a flow equation with a sink item that describes the characteristics of pressure-lime chart when the pressure is higher than the maximum condensate pressure. We have established a sink item to show the influence of accumulation of condensate liquid according to Duhamet Principle of Superposition, and introduced two coefficients for it: condensing strength RD and condensing relaxation time λD. This paper gives the principle and the quantitative expression of the well pressure influenced by condensate function in the flow equation. An analytical solution for an infinite system is obtained (constant rate). These results can be used to analyse the unsteady flow test of constant production.