Aiming at the present situation that there is no evaluation method for the green degree of fabricated steel structure buildings in China, the evaluation index system is established by understanding the construction characteristics of fabricated steel structure from seven aspects of material production, environmental harmony, safety, energy conservation, social function matching rate, fabricating technological level and using function. The main and objective weights obtained by the constant weight method and variable weight method are combined optimally by means of multiplicative synthesis and game theory to coordinate the subjective value of decision makers with the objective information contained in the data. The evaluation results are graded according to the comprehensive evaluation method of barrel theory to evaluate the green degree of fabricated steel structure. The study result shows that the grade of green degree of fabricated steel structure is dark green. The study is helpful to comprehensively understand the current situation of the impact of fabricated steel structure construction on the environment, which can provide the beneficial reference for evaluating the green degree of fabricate steel structure buildings.