The present invention relates to an environmental load assessment system, which is capable of efficiently and simply assessing an environmental load of a building in all stages (e.g., a planning stage, a design stage, a construction stage, and a use stage after construction) in doing executing a business by subdividing an assessment process, such as simple assessment in the design stage, detailed assessment in the start stage, and site assessment in the construction stage according to the progress of a construction project. The system of the present invention includes a DB server for storing material information, environmental load information, and itemized material information about a construction work and storing environmental load information about an energy source; a management server for assessing environmental loads based on simple assessment, a detailed assessment, and site assessment according to the progress of a target assessment construction project; and a UI server for providing user terminals with an execution environment so that the management server can be operated.