The invention discloses a system for detecting the temperature and depth of sea water by a fiber bragg grating. The system comprises a data processing device and an underwater fiber sensor chain which are connected through a transmission optical cable, wherein the data processing device comprises an optical fiber coupler connected with a light source and a wavelength detection module connected with the optical fiber coupler; the wavelength detection module is connected with a data processing module; and the optical fibber sensor chain is formed by connecting more than two temperature and depth sensing probes with the same structure in series through the transmission optical fiber and an optical fiber connector along the vertical direction. The optical fiber sensor chain generally comprises 4 to 8 temperature and depth sensing probes which are arranged at an interval of between 2 and 10m. The system has a simple structure, is safe and reliable, is convenient to use and can easily perform expandable measurement on depth and layer number; the detection signal is an optical signal; the technical problems such as water tightness, pressure resistance and the like of the conventional electric signal detection equipment are solved; and the measurement signal is stable and the system has obvious effect. The temperature and depth sensing probes have small volume and light weight, are connected through connectors and are convenient to transport, mount, debug and maintain.