Fast and reliable black start plays a key role in improving the ability of the power system to resist the risk of large-scale blackouts. For a black start with high voltage and long-distance transmission lines, it is much easier to cause phenomena such as self-excitation and power frequency/operating overvoltage, which may lead to black start failure and impact the reliability of the system’s restoration. Meanwhile, the long time needed to crank up the non-black start units will impact the speed of the restoration. This paper addresses the advantages of using a thermal power unit with a fast cut back (FCB) function as a black start unit, and studies the transient process of the FCB unit during the restoration. Firstly, key problems in the power system black start process are analyzed and a practical engineering criterion of self-excitation is proposed. Secondly, the dynamic model of the FCB unit is presented. Thirdly, the field test of the FCB unit load rejection and black start is introduced, which is the first successful field test of black start with 500 kV long-distance lines in China Southern Power Grid (CSG). Finally, the transient process of this test is simulated using the PSCAD/EMTDC software, and the simulation results accord well with the field test results, which verifies the correctness of the FCB model and the self-excitation engineering criterion proposed.