In order to decrease the output-voltage harmonic components of cascaded multilevel inverter, this paper proposes a novel staircase modulation strategy based on Karush-Kuhn-Tucker (KKT) conditions. With KKT conditions and rigorous mathematical derivation, this paper acquires the corresponding calculation method of conducting angles, and the proposed staircase modulation strategy can minimize the total harmonic distortion (THD) under arbitrary modulation index. Furthermore, this paper takes advantage of Newton-Raphson iteration method to obtain the key parameter in the aforementioned conducting angle calculation method. To overcome the deficiency of slow calculation speed of Newton-Raphson iteration method, this paper proposes the power function approximation approach to simplify calculation complexity. Finally, this paper undertakes simulation analysis and obtains the output voltage waveforms and their frequency-domain distributions under both fixed and varying modulation indices. Simulation results verify the effectiveness of the modulation strategy.