Abstract: The tightness of the double containment annulus is critical to the safety of nuclear power plants, and the test procedure and data analysis should be reliable and reasonable. The formulation of the pressure in double containment annulus versus time is deduced in this paper with the foundation of the quadratic function law of pressure difference and the rate of air flow, which is used to calculate the leakage rate of the annulus at specified time. The fitting of annulus leakage rate against pressure difference is resulted from the analysis of the property of reference point for the measurement of annulus negative pressure. The quadratic theory is verified with project measured data, and is optimised with a corrected pressure difference, which performs better, and is recommended in the calculation of annulus leakage rate test.