A design method of a three-direction double-loop strain sensor for two-dimensional (2-D) strain is proposed and analyzed. The strain sensor is made up of three sensing arms and each arm is made of fiber Bragg gratings (FBG) sensor surface glued on titanium alloy substrate. The substrate has double loops to improve the sensor’s measurement range. The strain sensor is unique in that by designing three single arms of strain sensors in different direction, a 2-D strain’s maximum strain value and direction can be precisely determined. In comparison with common unidirectional strain sensor, this has great value for structural health monitoring. With finite element analysis and measurement demonstration, it is shown the measurement range of the strain sensor is increased by seven times compared with normal FBG sensor. The strain sensor’s measurement range can reach 1% (±5000 μϵ) with accuracy of 0.6%. An accurate and real-time monitoring system based on the sensor is presented, which demonstrated the sensor’s fast speed and high reliability.