The present invention relates to an online computation main/standby dual system for a power system and a switching method thereof. An operation mode that two sets of online safe and stable computation systems simultaneously exists and are complementarily main and standby systems is formed in an I zone; when the main system carries out online computation and uploads index data to a national dispatching organization, the standby system simultaneously carries out online computation, but does not upload any computation result program; and when the main system is halted, generates the phenomenon of point loss or cannot normally carry out computation due to a certain reason, the main system stops working, the standby system is automatically switched into to upload the result program to the national dispatching organization, and vice versa. According to the present invention, free switching of double systems is implemented, reliability and accuracy of online computation are improved and the practical prospect is very wide.