In 1125, the Jurchens tore up the military covenant with the Northern Song, and launched southward. After the fall of the capital Kaifeng, Huizong, Qinzong, as well as a large number of clan and common people were captured by the Jurchens. Emperor Gaozong inherited the throne and created the Southern Song dynasty. In order to safeguard the continuity of the new regime, he and his prime minister had to face many political issues. Lü Yihao (1071-1139) was the key prime minister of Gaozong to overcome the crisis and safeguard the regime. In this paper, the discussion focuses on four issues: (1) describes the expansionist policy from the mid-Northern Song; (2) retrospectively examines Lü Yihao's family background, as well as his participation in civil examinations; (3) tracks the first three major stages leading to the officialdom of Lü Yihao; (4) provides a detailed analysis of the character of Lü Yihao. By the above discussion, this paper tries to sum up the requisite conditions for a wartime prime minister to have.