The testing device for the air permeability performance of the exterior windows was developed ,according to the national standard GB/T 7107--2002 and GJ/T 211--2007. With the device, air tightness of building exterior windows can be measured. With the testing data of this device, the uncertainty in measuring the testing results was evaluated in the article, and the principal influence factors were analyzed. It provided the basis for improving the accuracy of test.