Aiming at the lack of specific energy absorption area and components for flat truck cab, a method for improving the cab’s crashworthiness was proposed, which was realized by reallocating the power dissipation of each component. Referring to rule ECE R29, finite element method was used to simulate the pendulum collision process of some real vehicle to distribute the power dissipation by changing the thickness of components of cab. Living space of the cab after collision was evaluated to analyze the relationship between it and power dissipation, and experiment was carried out based on the analysis result to give some reasonable instruction. The analysis result showed that an appropriate increasing or decreasing of power dissipation of each component could optimize the crashworthiness of cab effectively. 