Embedded control systems normally have modes such as startup mode, normal mode and emergency mode. Program modes are expressed by the combination of value ranges of input parameters, which are input parameter expressions. Deriving program modes from code cannot only verify the consistency between the design and implementation, but also improve the accuracy of WCET estimation. Based on the analysis of source code, we propose a new method which can automatically analyze program modes. We achieve an input-dependent control flow graph (ICFG) by working on the control flow graph of the source code, then redirecting the control flows of nodes in loops and deleting input variables independent nodes. We construct a linear programming problem for each path of the ICFG and gain an input parameter expression for a potential mode if the problem has a solution. Expression results for a benchmark program show that the method is feasible and effective.