We present an open-loop adaptive optics (AO) system based on two liquid-crystal spatial light modulators (LCSLMs) that profit from high precision wavefront generation and good repeatability. A wide optical bandwidth of $300\mathrm{nm}$ is designed for the system, and a new open-loop optical layout is invented to conveniently switch between the open and closed loop. The corresponding control algorithm is introduced with a loop frequency (the reciprocal of the total time delay of a correction loop) of $103\mathrm{Hz}$ . The system was mounted onto a $\mathrm{2.16}\mathrm{m}$ telescope for vertical atmospheric turbulence correction. The full width at half-maximum of the image of the star α Boo reached $0.636\mathrm{arc}\sec$ after the open-loop correction, while it was $2.12\mathrm{arc}\sec$ before the correction. The result indicates that the open-loop AO system based on LCSLMs potentially has the ability to be used for general astronomical applications.