Scale constraint is a basic rule of the surface complex system on earth. For environmental impact assessment (EIA), scale constraint is hid in the experience of EIA professionals, or scattered among EIA technical guidelines; it is not a large concern for EIA professionals. This paper discusses the scale constraints of environmental impact, EIA, and the EIA technical frame. Results show that scale constraint of environmental impact responds to the hierarchical structures of human activities, environmental elements, and the influence of human activities on the physical environment. EIA is apparently restricted by spatial extent and resolution, temporal duration and frequency, and analysis scale, which includes the EIA technical models and the perception or the level of understanding about the environmental impact. The spatial, temporal, and analysis scales are independent, while being integrated in the process of EIA and restricting the EIA conclusion. Any EIA can be positioned in the three scales. Generally, the spatial data with low resolution may be available for EIA with large spatial extent, while spatial data with high resolution is required for EIA with small extent, so is the relationship between temporal frequency and duration. Furthermore, spatial, temporal and analysis scale restrict the key technical contents of EIA based on the officially published EIA technical guidelines both for project and for planning EIA. It proposes that the spatial and temporal scales of EIA subject should be the core scale, during the process of EIA, spatial and temporal extents may vary flexibly from large to small, while their resolutions may change from coarse to fine.