Surface control of large-aperture transmission optics in both on-line working, and off-line testing, states is perceived as one of the most important requirements, and the largest engineering challenge, in high-power solid-state laser facilities. Consequently, a surface control apparatus and method was proposed based on the self-flexible force-moment technology and principle of load linearity, respectively. A series of analyses were conducted using the proposed apparatus and method both mechanically and numerically. Furthermore, the principle of phase mismatch induced by distortion and second harmonic generation (SHG) efficiency was analysed theoretically. Finally, the trends of the surface RMS value and efficiency under different load regimes on certain loading states were deduced, and the trends in the best surface RMS value and efficiency of each loading state were analysed.