The steel pipeline system is a popular means of transporting natural gas. Most of the pipelines are buried under the sea or ground. Defects in gas pipelines such as metal corrosion, pitting, cracking, environmental electrochemical reactions, welding defects and damage caused by external forces can destroy the integrity of the pipeline and therefore need to be inspected regularly. Non-destructive testing based on the PIG (Pipeline Inspection Gauge) and the magnetic flux leakage (MFL) method is usually performed for gas transportation steel pipeline systems. In this paper, the authors will introduce a 20-inch PIG designed, manufactured and tested in the workshop with the API standard. The authors used finite element simulation to optimize the configuration of the magnetizer system for the PIG to obtain the best MFL signal. The PIG has also been built relatively completely and through testing, it has been well detected with defects with a depth of 12.5% of the pipe wall thickness and square defects of  ±15 mm in size, a depth of about 15% of the pipe wall thickness.