In order to solve the problem which ASON (Automatically Switched Optical Network) has long recovery time and high blocking rate when using traditional path calculation method, a new algorithm based on the optical power attenuation value is introduced. The value is used to determine the probability of the path of each link which would be selected and the reachable lists of each station, then a recovery path which consists of balance link traffic and shortest hops based on principle of the product of the maximum probability is selected. On-site test shows that the path calculated by this method can get a more balanced selection of each link, and also can reduce the rate of path blocking and guarantee the delay recovery time within 100 ms. This work is supported by National Natural Science Foundation of China (No. 50677047) and Natural Science Foundation of Hubei Province (No. 2015CFB563).