A building slab, as a main stress component, plays an important structural role. To better understand the environmental performance of various slabs and assist design decision-making considering both environmental protection and structural requirements, this paper evaluated and compared the EIs of two commonly used slabs (RC slabs and composite slabs) with various sectional forms from the perspective of mechanical features. The functional unit was defined as the cross-sectional area of a slab satisfying certain structural span and load requirements. Based on the LCA-based BEPAS model, the EI of slabs were divided into two safeguard areas (resource depletion and ecological damage) and then expressed by the EIV index in units of "yuan" using the willingness to pay (WTP) weighting method. Combining EIV and permitted load-span (q-L) curve, q-L-EIV interactive curves were developed to holistically illustrate the structural and environmental performance of slabs in a single diagram. A series of comparisons were conducted to demonstrate how the developed q-L-EIV interactive curve work to facilitate analysis and selection of slabs. Additionally, sensitivity analyses were performed to evaluate the effects of main parameters (i.e. steel ratio, steel strength, concrete strength and recycling rate) on EIA results of slabs. The research results indicated that there are large differences between the EIs of various slabs, and composite slabs were widely recommended in engineering applications from the view of environmental sustainability. Moreover, the proposed curve can serve as a reference tool of slab design to provide structure-environmental oriented implications in construction industry practice.