The line-tapered multimode interference couplers which are more compact than traditional multimode interference couplers had been widely used in integrated optical components. The self-imaging properties are the most important characteristics for their applications in integrated optical components. Based on the self-imaging of multimode interference couplers, we analyze the general self-imaging properties in line-tapered multimode interference couplers in detail, and find the positions of images which directly relate to the position of input field, the position number, the initial width of line-tapered multimode interference couplers and transition ratio. The analytical expression of image positions and some characteristics are summarized. Finally, the guide-mode propagation analysis and finite-difference beam propagation method (FDBPM) are used to confirm the analytical results. These conclusions can be used to exactly get the positions of images without complex calculation, which provide a theoretical basis and design reference of integrated optical components with line-tapered multimode interference couplers.