The loss of excitation (LOE) is one of the main faults of the synchronous generator, which may cause serious damages to generator and makes the power system unstable. The main problem to distinguish between LOE and stable power swing (SPS) is a time delay, which is a main part of the detection time in all recent protection methods. This study presents a new method for LOE detection in the synchronous generator, which reduces time delay by half. The proposed method exploits a new index which is based on the variation of the generator output reactive power. It is a combination of the first- and second-order derivative of the generator output reactive power. In this method, after the generator losses its excitation, the proposed index will become and remain negative for a while. To prevent mal-operating under slowest power swing condition, the time delay of 0.85 s is used. The performance of the proposed method is evaluated under different conditions. The simulation results and experimental tests confirm the accuracy and fast operation of the proposed method.