A flexible assembly system for multi-product production and a method of reconfiguring a production line for multi-product production. A method of designing a production line includes configuring reconfigurable assembly modules (RAMs) for each production process of the production line using components included in a production line configuring tool, verifying a production amount of each of the RAMs using a production line simulator, designing the production line using RAMs with a production amount greater than or equal to a threshold value, and transmitting design information of the production line to an onsite production line installer terminal.