An optical fiber plane strain sensor with droplet-like structure encapsulated by polydimethylsiloxane (PDMS) was proposed to monitor the deformation of thin slice workpiece.The sensor was formed by bending a piece of standard single mode fiber (SMF) to form a whispering wall structure,shaped like a droplet,and was encapsulated in a square mold made of PDMS.The magnitude and direction of the external strain on the workpiece were obtained by monitoring the change of the output spectrum wavelength of the sensor in the experiment.Experimental results showed that when the sensor was strained along the small axis of the droplet-like structure,the output spectrum was blue-shifted with the sensitivity of －0.108 nm/με.While the output spectrum was red-shifted when the sensor was strained along the large axis with the sensitivity of 0.084 nm/με.The experiment also studied the condition when the force direction was 45° with the short axis,which provided a reference for judging the strain direction,and temperature sensitivity of the sensor was －1.557 nm/℃.Therefore,the proposed optical fiber plane strain sensor could not only detect the magnitude of strain,but also distinguish the directions of strain through strain sensitivities,so as to achieve the purpose of plane strain measurement.The sensor will have a good application prospect in such fields as coating,bending and so on.