Abstract: In order to clarify the effects of soil salinity on the insect-resistance of boll in transgenic Bt cotton, potted plants of two Bt cotton cultivars Xinmian 33B (salt-sensitive) and Zhong 07 (salt-tolerant) were exposed to five levels of soil salinity (0, 0.15%, 0.30%, 0.45% and 0.60%). The results showed that Bt protein content of boll shell decreased with increasing soil salinity. Compared with the control (0% soil salinity level), the Bt protein content of boll shell decreased significantly when the soil salinity level was above 0.15% for Xinmian 33B and above 0.30% for Zhong 07. The reduction extent of Bt protein content of boll shell at 30 days post anthesis (DPA) was greater than that at 10 DPA under the same soil salinity level. Significant reductions of soluble protein contents, nitrate reductase (NR), and glutamate pyruvate transaminase (GPT) activities were observed when the boll shell Bt protein content was significantly reduced. The content of free amino acid, protease, and peptidase activity of boll shell significantly increased when the soil salinity level was above 0.30%. In conclusion, soil salinity affected boll shell nitrogen metabolism and reduced Bt protein synthesis. Middle and high soil salinity levels could enhance decomposition of Bt protein, which further decreases the expression level of insecticidal protein.