Abstract: This paper establishes a mathematical model of pressure drawdown test for the wells with variable wellbore storage effect.Applying the Laplace transform and numerical inversion method,a new pressure drawdown welltesting interpretation chart with variable wellbore stotage coefficient is developed,which is a combination chart of pressure and pressure derivative taking βDe2S as the changing perameter,whose abscissa is tD/βD(tD is dimensionless time,βD is dimensionless attenuation factor),whose ordinate is PD(dimensionless pressure) and P’D·tD/βD(P’D is dimensionless pressure derivative).It has many advantages differ from that of Gringarten chart.That the early-time portions in the said chart is not the straight lines with a slope of 1,but a series of arc curves whose dip angles are more than 45° coincides with the real situation.This chart is easy to use,the interpretated parameters are accurate and reliable.The cartographic way,special feature and operating method of the above-mentioned chart are introduced in this paper.