The invention relates to a fiber temperature sensor, a fiber thereof and a fire alarming device using the sensor. The fire alarming device comprises the fiber temperature sensor, a control system for processing signals detected by the fiber temperature sensor and an alarm system connected with the control system. The fiber temperature sensor comprises the fiber having a bendable section. The fiber core for transmitting light waves of the fiber is provided with a light leakage hole for enabling a part of the light waves to be leaked at the bendable section of the fiber, and at the place of the same curvature radius when the bendable section bends, the light leakage hole faces to the direction same with the direction of the curvature radius or opposite to the direction of the curvature radius. The fiber temperature sensor further comprises a fiber deformation device for enabling the curvature of the bendable section to change correspondingly according to the change of the temperature. The light leakage hole can enable the light intensity of the fiber to change obviously when the bending condition of the fiber changes, thereby realizing improvement of system detection sensitivity, reducing fiber cost, and reducing the cost of matched wavelength processing equipment; and production, debugging and maintenance are done conveniently.