A basic input/output system (BIOS) capable of supporting multi-platforms and a constructing method thereof are provided. In the method, a plurality of segment modules is provided, and each of the segment modules includes more than one BIOS module. A module header is established for each of the BIOS modules and records an application platform identifier (ID) of an applicable platform of the corresponding BIOS module and module type data of the corresponding BIOS module. The segment modules are then integrated according to a design structure of the BIOS. The segment modules are classified into a main system module, a plurality of slave segment modules, and a reset entry segment module. When the reset entry segment module obtains a platform ID of an electronic equipment currently configured with the BIOS, the main system module pre-stores an execution sequence according to the platform ID, and sequentially runs the BIOS modules matching with the platform ID.