The invention discloses a transformer neutral point overvoltage assessment method inside a transformer substation. For a transformer with an ungrounded neutral point, the neutral point is protected bybeing connected with an arrester in parallel through a protective gap; when a circuit has a one-way ground fault, is in open-phase operation and has lightning stroke overvoltage, the neutral point ofthe transformer with the ungrounded neutral point can generate large overvoltage, and a protective gap is caused or the arrester is made to act. According to a transformer substation electrical mainwiring diagram and a tower arrangement diagram of all outgoing lines, a system model is built in simulation software, modeling is performed on an electric transmission line, instation GIS equipment and transformers, and the transformer neutral point overvoltage assessment method inside the transformer substation is put forwards. The protective gap selection method based on an overvoltage assessment result and the neutral point insulation level assessment method are put forwards. The purposes of calculation of transformer neutral point overvoltage calculation inside the transformer substation and selection of the insulation level and the corresponding protective gap are achieved, and the safe and reliable operation level of the transformer is increased.