In order to evaluate the security of the 500 kV terminal substation，an engineering risk evaluation algorithm with an eye to its characteristics is proposed.Several major risk indices are taken into consideration，including grid security，failure of power supply，device damage and social influence.Then calculation models for various risks are developed based on the analytical method.A certain500kV terminal substation rebuilding scheme in Guangdong grid is analyzed as an example.The risk of it is evaluated，and the risk changes before and after rebuilding are compared.The sensitivity of several major risk parameters is analyzed.The results show that the device damage and the failure of power supply are the major risk factors.After terminal substation rebuilding，the risk of device damage is considerably decreased and the risk of power supply failure is slightly increased，hence the reduced comprehensive risk.