The newly developed lightweight ultra-high-performance concrete (LUHPC) exhibits exceptional properties, including low density, high compressive strength, and reduced autogenous and drying shrinkage. It is particularly suited for structural strengthening of existing concrete bridges requiring immediate load-carrying capacity enhancement. Nevertheless, the interfacial bond performance between LUHPC and the substrate (i.e., existing concrete) constitutes a critical determinant of reinforcement effectiveness and long-term structural integrity. In order to obtain the bonding performance at the interface of LUHPC-reinforced concrete (NC, C50) under severe cold and high-salt conditions, a series of tests including composite salt erosion, freeze-thaw cycles, and composite salt-freeze-thaw cycles were conducted on the LUHPC-NC interface. Through mechanical tests, the splitting tensile strength and oblique shear strength of the reinforced specimens after erosion were obtained to explore the influence laws of the complex environment on the bonding performance of the interface. Furthermore, three different interface treatment methods were implemented to assess their influence on bond strength. The developed interface bonding strength degradation model and service life prediction model were employed to estimate both the residual interfacial bond strength and the maximum service life of the reinforced specimens subjected to varying extents of corrosion. The test results indicate that, the LUHPC-reinforced NC specimens exhibited failure modes involving both the matrix and the interfacial bond region under splitting tensile and oblique shear loading conditions. With increasing severity of environmental erosion, the degradation rates of both splitting tensile strength and inclined shear strength accelerate in C50 reinforced normal-strength concrete (NC) specimens. After 100 cycles of chloride-freeze-thaw exposure, the splitting tensile strength and inclined shear strength of LUHPC reinforced NC specimens are 1.79 and 1.62 times those of C50 reinforced NC specimens respectively. These results indicate that LUHPC exhibits superior interfacial resistance to environmental degradation compared with C50 concrete. Furthermore, higher interface roughness significantly enhanced interfacial bond strength. After 100 cycles of chloride-freeze-thaw exposure, specimens with grooved interfaces exhibited splitting tensile and oblique shear strengths 1.94 and 2.26 times greater than those with untreated interfaces, respectively. The calculated values of the model were in good agreement with the test values, with an error within 7%. These findings provide valuable insights for the reliable application of LUHPC in structural strengthening projects and contribute to ensuring the post-reinforcement integrity of concrete components.