A shipborne expendable optical-fiber bathythermograph comprises an all optical fiber probe, transmission optical fibers and a deck unit, wherein the probe measures the seawater temperature through LPG (long period fiber gratings) and measures the seawater depth through FBG (fiber bragg gratings), and seawater temperature and depth profiles are measured independently at high sensitivity; the deck unit comprises a broadband light source, a dynamic fiber grating fast and high-precision demodulation module, an optical fiber isolator, a first optical fiber coupler, a second optical fiber coupler, an upper fiber shaft and a telescopic support mounted on the board of the stem of a working ship. The thermograph solves the problems of cross sensitivity and sensitization of a sensor, the sensitivity of temperature and depth measurement is improved, errors caused by enameled wire channels are overcome, the signal transmission rate and the stability are guaranteed, single light path cascade of FBG and LPG is realized, a light path structure is optimized, the spectral region is allocated reasonably, temperature and depth synchronous detection is realized, the thermograph is simple to manufacture, the probe is low in cost, and a new technical means is provided for detection of the seawater temperature and depth profiles.