In view of the limitations of the current fiber coupler quality detection technology, that is, the traditional destructive dissection method leads to sample loss, and the lumped optical power test is difficult to locate the spatial distribution of internal defects. An innovative detection method based on the collaborative analysis of thermal excitation and Optical Frequency Domain Reflection (OFDR) strain measurement is proposed in this study to achieve millimeter-level spatial analysis of the deformation characteristics of the inner fuse cone region, the geometric symmetry of the rubber layer and the interfacial stress field of the fiber coupler. The quantitative defect evaluation system based on thermal strain gradient can directly provide data support for process optimization, thereby improving device reliability and reducing failure risk. This method breaks through the technical bottleneck of traditional trial and error process optimization and lays a technical foundation for the long-term stable operation of optical communication and optical sensing systems.In this study, OFDR distributed sensing technology combined with a wide temperature domain stepped temperature loading (-35 ℃ to 65 ℃, at intervals of 10 ℃) was used to invert the mechanical behavior of the internal structure of the device. In the experiment, each temperature point was kept warm for 30 minutes to ensure the uniform distribution of internal temperature, and the multi-point temperature sensor was used to monitor the temperature difference between the shell and the rubber layer in real time. Based on the OFDR strain measurement system, the frequency shift of Rayleigh scattering spectrum along the fiber is accurately captured, and the high-resolution strain distribution curve is demodulated by the linear relationship between the frequency shift and strain. Further, the characteristics of the internal melt cone change, the symmetry of the rubber layer and the interface stress concentration are retrieved by the curve characteristics and the trend of change, so as to achieve the internal structure characterization of the device.This study systematically summarized the key characteristic parameters of the three groups of samples: The length of the cone zone, the maximum peak-valley difference and the asymmetric index, as shown in Tab.1. The experimental results show that the results of the first device reveal the influence mechanism of the thermal strain of the device when the coefficient of thermal expansion of the rubber is mismatched, as shown in Fig.4. Further analysis shows that the asymmetric characteristics of the thermal strain distribution curve (Fig.6) can effectively characterize the process defects of the coating uniformity of the package adhesive layer. The asymmetric index of the third device is as high as 45.7%, revealing the typical defects of the uneven thickness of the adhesive layer. In addition, the study also found that the low temperature environment has a particularly significant impact on the thermal strain of the device. For example, when the temperature is lower than -35 ℃, the strain of the rubber layer at both ends of the melt cone of the third device has an additional double peak rise phenomenon, and the change rate of the peak and valley value also changes sharply, and the strain change rate of the left peak and valley value surges to 4.2 times of that before the change. This phenomenon reveals that too low temperature will not only lead to a sharp decline in the mechanical properties of the adhesive, but also affect the stability of the cone region structure, and then affect the optical properties of the coupler.Through the analysis of thermal strain distribution in a wide temperature range, the correlation model between the internal structure of the fiber coupler and the thermo-mechanical response is established successfully. Based on the analysis of the characteristics of the thermal strain distribution curve and the change law, the feature identification and defect location of the internal structure of the coupler are realized. The strain results of the three devices at low temperature are analyzed emphatically and the meaning of the peak and valley values in the curve are explained. The internal structural parameters of the coupler, such as the thermal expansion difference of multilayer rubber, the thickness uniformity of the rubber layer and other potential defects, are determined and the corresponding optimization suggestions are given. This method can also be used to characterize the internal structure of passive fiber devices such as polarization-maintaining fiber couplers and wavelength division multiplexers.