Frequency selective surfaces (FSSs), modern artificial materials, show great potential in engineering applications due to their excellent frequency selection capabilities. In this paper, we introduce a flexible strain sensor based on FSS reflection characteristics, which can be well conformally attached to the surface of an object and bear mechanical deformation from a certain load. When the FSS structure changes, the original working frequency will be shifted. By measuring the difference in electromagnetic performance, the strain degree of the object can be monitored in real-time. In this study, we designed an FSS sensor with a working frequency of 31.4 GHz and amplitude that reaches -35 dB that exhibits favorable resonance properties in the Ka-band. The quality factor of FSS is 16.2, which indicates that the sensor has excellent sensing performance. The sensor was applied in the strain detection of a rocket engine case through statics and electromagnetic simulations. The analysis showed that the working frequency of the sensor shifted by approximately 200 MHz for 1.64% radial expansion of the engine case and the frequency shift exhibits an excellent linear relationship with the deformation in diverse loads, so it can be used for accurate strain detection of the case. Based on experiments, we carried out the uniaxial tensile test of the FSS sensor in this study. The sensor's sensitivity was 1.28 GHz/mm when the FSS was stretched by 0-3 mm in the test. Therefore, the FSS sensor has high sensitivity and strong mechanical properties, which verifies the practical value of the FSS structure designed in this paper. It has a broad development space in this field.