A new optical fiber sensor developed for simultaneously measuring the seawater temperature and salinity is described. The sensor respectively exploits the displacement of the optical absorption edge in semiconductors under the influence of temperature variation as a result of temperature induced energy shifting of conduction band extrema, and the beam deviation caused by the optical refraction at the receiving end face due to the salinity of seawater. The measured optical signals are reflected and transmitted through a sequentially linear arranged fibers array, and then the light intensity peak value and its deviant are detected by a charge coupled device (CCD). The sensor probe is composed of an intrinsically pure GaAs single crystal,a right angle prism, a partitioned water cell, the emitting fiber with an attached self focused lens and the linear arranged receiving fibers array. Theoretical analysis and simulations verify the feasibility of the proposed system.