To determine the calculation formula for subcritical flow big leakage,its character was analyzed in gas pipelines.Based on the small hole model and the pipeline breaking model,a big nozzle model was build and a calculation formula was given for the leakage intensity.Tests were carried out to measure the leakage intensity under different pressure and aperture and to verify the accuracy and applicability of the calculation formula.It is shown that the calculated result agrees well with the measuring one when the nozzle size is small.