Steering knuckle has a direct influence on the vehicle's operating stability and driving safety. In order to meet the requirements of lightweight of steering knuckle and automobile dynamics simultaneously, interpolation model of SIMP(solid isotropic material with punishment) density function and weighted compromise optimization method are applied to define synthesis objective function and carry out multi-objective topology optimization of a solar car's steering knuckle. The purpose of the optimization is to make the first order vibration frequency and the stiffness of the static working condition reach the comprehensive optimum value. After optimization, the first order vibration frequency of the steering knuckle is increased by 76.87%, the stiffness under the static working condition is increased by 90.25%, the mass is reduced by 44.47%, and the effect of lightweight is remarkable.