<p>The full life cycle environmental assessment method is widely applied in infrastructure projects such as buildings and highways. However, its application to bridge life cycle assessments remains challenging. This paper addresses this gap by conducting a comprehensive environmental impact study throughout the entire life cycle of bridges. A foundational framework is proposed for evaluating the environmental impacts of bridge life cycles, dividing the process into four phases: raw material production, construction, operation and maintenance, and demolition and recycling. The evaluation system is applied to a newly constructed bridge, comparing two distinct construction methods—prefabricated small box girder and cast-in-place large box girder—to provide data for life cycle impact analysis. Four key factors—transportation distance, maintenance frequency, recycling coefficient, and material usage—are incorporated into the analysis. Sensitivity analysis, with a 10% fluctuation range for each variable, reveals that the initial phase (raw material production and processing) has the most significant environmental impact, particularly on human health, followed by resources and energy, and finally ecosystems. Among the variables, material usage during the initial phase shows the greatest sensitivity, followed by recycling efficiency in the demolition phase, repair frequency during operation, and transportation distance during construction. Regarding construction methods, the prefabricated method results in a higher level of pollution compared to the cast-in-place method. Based on these findings, a carbon emission reduction strategy is proposed to support green and sustainable development, aiming for carbon neutrality. The study highlights the importance of selecting low-carbon, sustainable materials in the design phase to minimize environmental impacts over the life cycle.</p>