Based on the TOE framework theory, the complexity evaluation elements of megaprojects were identified and a complexity measurement system was established through a literature review and expert interviews. Then the reliability, validity, and factor analysis of 94 questionnaires were tested to set up the framework of complexity factors. Moreover, on the basis of the ANP model, the weight coefficients of each index were calculated, and the complexity calculation formula for megaprojects was put forward and then verified by cases. This paper theoretically clarifies the definition and connotation of megaprojects and their complexity as well as establishes a breakthrough quantitative evaluation system for the complexity of megaprojects. It realizes objective appraisal of project complexity and provides an efficacious method for project managers to accurately grasp the project complexity.