The transmission tower is important structure equipment in the power system. The bolts which connect the angle steel together may loose under the cycle loading. As a further effect, the dynamic behaviors of the tower may change and lead to failure. In this paper, the finite element model of a typical 500 kV transmission tower which considers the concerned region actual connections of bolts was established. And modal analysis was conducted on two different models to obtain the influence of the bolt looseness on the natural frequencies. After that, a method based on the modal analysis and the radial basis function (RBF) neutral network was used to identify the location and the severity of bolt looseness in a lap plate model. The results show that the bolt looseness of concerned region has obvious influence on the high order mode frequencies and the identification method can be applied in the looseness detection of transmission tower in further work.