Abstract: The reliability of pipelines is affected by a large number of factors. In order to investigate the influence laws of third party damage factors on the reliability of pipelines, a quantitative calculation model was established by combining the impact probability caused by the third party damage and the pipeline failure probability under the effect of impact force. Then, the fault tree of third party pipeline damage was built up by means of the fault tree analysis method to determine the type and logic calculation relation of the factors that influence the third party damage activity. And the method for determining the probability of basic event in fault tree was studied to calculate the probability that the pipeline is impacted by the third party damage activity. Under the effect of the impact force caused by the third party damage, the material strength of pipe itself generates resistance. Based on the interaction between impact force and resistance, both punch-through failure model and dent-gouge failure model were studied and their limit state equations were confirmed. By virtue of the limit state equations, the influential degree of third party damage on pipeline reliability can be calculated quantitatively. Finally, a domestic natural gas pipeline was taken as an example. Its parameters which were needed for the calculation were collected and the probability of pipeline failure caused by the third party damage was calculated.