During power system restoration， the maximum restorable load that a substation can pick up at each time should be determined. Frequency constraint and steady-state voltage constraint are normally the main focus in the previous researches. In this paper， to determine the maximum restorable load， the transient voltage security is involved in the mathematical model as a constraint. To solve the proposed model， firstly， a two-element table describing the requirements on transient voltage security for each bus is presented. Then differential-algebraic equations are established to investigate transient voltage security. In order to improve the convergence speed， some system components， such as generators， inductive motors and network， are simplified within certain precision. The modified dichotomy theorem including constraint margins is proposed to solve the established mathematical model resulting in higher convergence speed. Its validity and effectiveness are demonstrated by simulation results of IEEE 14-bus testing system and practical Shandong power system in China.