When analyzing the mathematical model of the unsteady
flow in the tight gas reservoir which has start-up pressure
gradient ,its usual method is using Green Function to obtain
the solution in the Laplace Space , yet which cannot be
inverted to real space to acquire the analytical solution.Based
on the nature of Laplace transformation and inverse Laplace
transformation , this paper introduces the additional function
and obtains the concise analytical solution in the real space.
The comparison of the results of analytic solution and
numerical inversion shows that , the calculation accuracy is
within 5 percent , and thus it can be applied to the
engineering.