Release parameters of natural gas transmission station (NGTS), such as release rate, released mass, etc., are important for predicting dispersion range of gas jet and flame impingement radius. Through the intervention effects of safety interlock system (SIS) which includes automatic isolation system and blow-down system, the release parameters change with the time as the isolated process unit depressurizes in release accident. The proposed model in this study takes the effects of SIS into consideration for calculating the parameters of unsteady release. A case application confirmed the effects of this model. The results support quantitative consequence assessment and accident emergency response.