Light-activated shape memory polymers (LaSMPs) exhibit stiffness variations when exposed to ultraviolet (UV) lights. Thus, LaSMP could manipulate structural frequencies with UV light exposures when laminated on structures. This study aims to experimentally demonstrate the effectiveness of LaSMP frequency control of a flexible beam. The natural frequency of a three-layered Euler–Bernoulli beam composed of LaSMP, adhesive tape, and the flexible beam is analyzed and its frequency formulation exhibits the LaSMP stiffness influence. Since the LaSMP adopted in this study is a new spiropyran-based composition—Sp3/EVA_4, a generic Young’s modulus model is proposed and then simplified to model this new LaSMP composition. To guarantee a homogenous light field, light intensities on the UV surface light source at different positions are tested. The temperature change of the LaSMP sample under UV exposures is also measured. The time constant and the threshold intensity of the reverse reaction are measured. LaSMP Young’s modulus variation is tested with a uniaxial tension experiment. The constitutive model of LaSMP’s Young’s modulus is validated by experimental data. With these preparations, the LaSMP laminated flexible beam model is exposed to UV lights and its natural frequencies are identified with data acquisition and analysis system. Then, natural frequency variations of 25%, 50%, 75%, to 100% exposure areas are also evaluated. The maximum natural frequency variation ratio achieves 9.7%; theoretical predictions and experimental data of LaSMP natural frequency control are compared very well.