The environmental impacts of the construction stage should be considered since a large amount of building materials are used to construct a building at this stage. Studies on the improvement of construction techniques or the application of newly developed construction methods for reducing the environmental impacts are relatively scant compared to other topics of studies. This study aimed to assess and compare the environmental impacts of the ordinary solid slab, the flat plate slab and the voided slab system during the construction phase. A process-based quantitative model was adopted to evaluate the environmental impacts and the comparative results were analysed to demonstrate the significant characteristics of the environmental impacts of the construction of slab in a building. The assessment results show that the environmental impacts from the ordinary solid slab are the highest and the voided slab system is the lowest among three slab systems. As the slab system of the studied building was replaced, it was shown that the environmental impact indicators showed the decreased tendency. Based on the results of environmental impact reduction from the ordinary solid slab, the flat plate slab and the voided slab system, the voided slab system would have the least environmental impact in all indicators.