What-if simulation methods and systems are provided. A design coding in HDL (Hardware Description Language) and a simulation result corresponding to the design are provided. A what-if design scope and a what-if time window are received. A portion of the design is extracted from the design according to the what-if design scope, and primary input signals are determined during the extraction of the sub-design. Then, what-if simulation data is extracted from the simulation result according to the primary input signals and the what-if time window. A what-if test bench is generated according to the what-if simulation data, wherein the what-if simulation data is read, and the signal values are fed to a simulator according to the what-if test bench.