In the era of big science, the construction of large science projects is becoming more complex. Designers have to comprehensively consider factors such as instrument performance, technical reserves, funding, risks, and the environment to make a reasonable decision. On the basis of domestic and international astronomy, this paper sorts out the process of the formation of the Five-hundred-meter Aperture Spherical radio Telescope (FAST) concept and the decisions made by the FAST team, including the Chinese concept of the large telescope, the proposal of the prototype, and the adoption of the active reflector, etc. We also discuss the process of decision-making. FAST was born in the process of interaction and integration between Chinese and international astronomy development, and realized the transformation from following up to taking the lead. It can provide a reference for constructing future large science projects with limited foundations in terms of technology and funding.