Although straw decomposition is important for ecosystem fertility and carbon balance, influence of ultraviolet-B (UV-B) radiation and nitrogen (N) deposition on this process is unclear. In this study, UV-B-exposed rice straw was decomposed under different N addition treatments for 15 months to investigate the indirect effects of UV-B radiation on straw chemistry and direct effects of N deposition on decomposition. UV-B exposure during rice plant growth changed the rice straw chemical composition, increasing the concentrations of acid-insoluble fraction (AIF), acid-soluble fraction, and UV-B-absorbing compounds. High N content had a negative effect on decomposition of rice straw exposed to enhanced and ambient UV-B radiation. Both AIF concentration and FTIR peak intensities indicated that lignin in rice straw was selectively preserved following N addition and UV-B radiation, reducing straw decomposition rate, which corresponded to lower activities of lignin-degrading enzymes in the later stage of decomposition. Thus, enhanced UV-B radiation during rice plant growth produced more recalcitrant substrates (lignin) and N reacted with lignin to produce more resistant compounds, further decreasing straw decomposition rate. UV-B radiation during plant growth and N deposition inhibit litter decomposition in agroecosystem, and their effects should be considered when establishing biogeochemical models in response to global changes.