To improve the anti - photoageing property of B72, a hindered amine light stabilizer having a high molecular weight, H - 622, was selected to modify B72. The color change, weight loss and other parameters of the ageing process were monitored and the photo - stabilization mechanism was investigated. The experiment results showed that stability of the modified material was greatly enhanced after modification with the hindered amine light stabilizer H - 622. The infrared spectrum, contact angle, weight -loss, hardness and other parameters of the modified material changed much less than did B72 itself after aging. Moreover, the color change of the modified material was negligible after about 500 hours of ageing. It is proposed that the mechanism of stabilization involves absorption of light by the hindered amine and generation of nitroxyl radicals, which can trap free radicals produced by degradation of B72, resulting in inhibition of the B72 degradation reaction. Therefore, it seems practical to modify B72 with hindered amine light stabilizer H - 622, thus providing a new method to extend the lifespan of B72.