The thermal output effect of fiber Bragg gratings(FBG) in low-temperature environments is significant and exhibits strong nonlinear characteristics, which creates difficulties for strain and temperature testing of the common-bottom structure of rocket cryogenic storage tanks. Based on fundamental sensing principles, this study analyzes the thermal output sources of bare FBG and strain-sensing FBG. A decoupling method for the thermal output data of FBG was proposed for ground mechanical tests of rocket low-temperature storage tanks. An experimental system for calibrating the thermal output of FBG in extremely low temperature environments was built, and the thermal output characteristic curve of FBG in the liquid helium room temperature range was obtained. The thermal response relationship between strain-sensing FBG bonded to the same material as the common-bottom structure and bare FBG was established. The application of FBG for online monitoring of strain and temperature in the common-bottom structure of rocket cryogenic storage tanks was carried out. By constructing a strain and temperature sensing network based on FBG, real-time temperature and strain data of the common bottom structure have been obtained. The results indicate that this study can effectively decouple the thermal output data of FBG in extremely low-temperature environments, accurately obtain the strain and temperature distribution of the common-bottom structure of rocket cryogenic storage tanks, and achieve good agreement with strain test data from low-temperature resistance strain gauges, with a correlation correlation exceeding 0.997. This method realizes all-fiber online testing in extreme environments, offering high deployment flexibility and sensor survival rate. It provides a case reference for the large-scale application of fiber optic sensing technology in structural cryogenic testing scenarios, and lays a foundation for the engineering implementation of fiber optic onboard testing.