In the present study, green synthesis of silver nanoparticles (AgNPs) was conducted to prepare a preservative solution for organic bamboo straws. Spherical AgNPs were prepared with the help of ultrasound waves by inducing a reaction between silver nitrate (AgNO3) solution (0.05 M) and tea leaf extract (Camellia sinensis). Spherical AgNPs with an 11–28 nm size were formed easily in the extract at ambient temperature. The as-prepared AgNPs–chitosan solutions exhibited excellent antifungal activity against Aspergillus flavus fungi, which were isolated from bamboo straws collected from the Nam Dong district of Thua Thien Hue province in Vietnam. Therefore, the AgNPs–chitosan solution can be used as a bamboo-straw-coating material to inhibit the growth of A. flavus fungi. The optimized concentrations of AgNPs and chitosan to achieve the antifungal activity of AgNPs–chitosan solution on bamboo straws were 40 parts per million and 1.5%, respectively. Thus, coating bamboo straws with this solution can be proposed as an additional treatment to increase their postharvest quality. Moreover, AgNPs–chitosan solution can be used to protect bamboo straws from fungal infections, thereby prolonging their shelf life.