To address the issue of low-range resolution caused by bandwidth limitation in terahertz imaging, this article proposes a multiband stitching method for the linear frequency-modulated continuous wave system. The method enhances the conventional time-domain synthetic bandwidth algorithm by incorporating a non-linear compensation algorithm based on a reference signal. This method ensures frequency and phase continuity for each frequency band while rectifying non-linear effects, thereby eliminating the need for further frequency shift and phase compensation. Experimental results using a linear FM imaging system with five frequency bands ranging from 0. 11 to 0. 75 THz show that the proposed method can significantly improve the longitudinal resolution of teraherz imaging and improve the accuracy of material dielectric constant measurement, and can be applied to the field of materials testing in the future. In addition, two surface step samples and an internal defect sample with an adhesive multilayer are designed for imaging. The imaging results reveal that the fused signals contain more detailed information compared to individual bands, enabling clearer separation of each layer within the samples. Additionally, steps with a height difference of only 0. 2 mm can be distinctly distinguished, which can meet the demands of high-precision non-destructive testing. Keywords:terahertz imaging; linear f