With the stability, high power generation and low environmental impact of offshore wind resources, offshore wind power is being used as the core power supply to promote the development of global low-carbon economy in the future. Firstly, combined with the characteristics of offshore wind power projects, this paper establishes a reasonable sustainability evaluation system from economy, reliability, environmental protection and safety. Then, in this paper, the mixed-cross weighting based on the coefficient of variation method-order relation method is constructed to determine the weight of project sustainability evaluation indexes. Finally, the function interval and form of the common origin gray clustering evaluation method are modified to determine the sustainability level of the projects. In order to verify the scientificity of the sustainability indexes and the accuracy of the evaluation model, combined with the case background of S offshore wind power project in J Province, the results show that the index that has the greatest impact on the sustainability of offshore wind power is the group participation of projects. At the same time, the improved evaluation method overcomes the problems of the inaccuracy of the rate change of the gray-crossing and membership degree of the common origin gray clustering function and makes the evaluation results more reasonable and authentic. This paper provides a reference for the investment decision of the offshore wind power project to be constructed.