This paper describes the applications, principle and common packaging method of existing fiber Bragg grating (FBG) testing technology. The applicability of FBG to strain detection of seasonally frozen subgrade which is a concealed work is explored. The main works are as follows: (1) Based on the method of embedded-beam for the testing technology of FBG, two ways for the packaging and layout of sensors are introduced taking account of the difference of testing objects, namely the shallow-setting semi-flexible feeler rod for short distance and the deep-setting semi-rigid probe for long distance. (2) To meet the need of tests in field, the systems of power supply, fiber optic sensors, data collection and analysis are compared and selected. (3) Two sites of seasonally frozen subgrade are chosen and a one year observation is conducted. The results indicate that FBG has a high survival rate within the temperature range from -20 ℃ to 30 ℃. The system of strain detection for subgrade in seasonally frozen soil regions presented and achieved here has a high testing sensitivity and reliability. The system provides a new technical means for testing physical indices of concealed works.