A heavy-duty truck cab was analyzed and lightweight designed by using topological optimization method. Performance of the initial cab and lightweight structure was compared. It indicated that the front pendulum impact property of the cab and roof static pressure performances of the high-roof cab were improved when structural strength, rigidity and low-frequency natural vibration characteristics of the cab were not lower than the initial cab. Meanwhile, structural mass of the cab was reduced by 46 kg without changing of cab materials. A good lightweight design effect is acquired.