Extensive studies have been performed on power-split hybrid electric vehicles (PS-HEVs) in the past two decades. Despite of the fact that their excellent fuel economy performance in city driving conditions has been witnessed, a bottle neck for further improving the fuel economy of PS-HEVs has been encountered due to the inherent engine-generator-motor power circulation of the power-split system in medium-low speed cruising scenarios. To further realize the fuel economy potential of PS-HEVs, in this paper, an optimal control based cruising problem is constructed. Due to the discontinuity of this problem, Legendre Pseudo-spectral Method and knotting technique are employed. By observing the optimization results, two different periodic cruising strategies can be extracted to control the charge and discharge operation of the energy storage system in realtime. By comparing the proposed strategies with a constant speed cruising strategy, a significant improvement can be observed and the effectiveness of proposed cruising strategies is validated.