When liquid crystal spatial light modulator is used for real-time adjustable beam shaping in inertial confinement fusion (ICF), Gamma curve with linear variation is of great significance. We present a new adjustment method for Gamma curve of liquid crystal spatial light modulator to vary linearly based on the existing hard sledding and time-consuming adjustment method. Discrete points for which gray values are used as abscissa and power values as ordinate are fitted to curve equation by Lagrange interpolation. Depending on the curve equation, the new gray values corresponding to the power values on the target Gamma curve are obtained with the gray value interval of 1 by Bolzano dichotomy. Then, replacing the original gray values with new ones makes the Gamma curve vary linearly. Using the new adjustment method for beam shaping in high-power laser amplifier facilities, super-Gaussian flat-top beam with a small near-field modulation and long-term stability is obtained.