Vehicle platooning has the potential to significantly improve traffic throughput and reduce fuel consumption and emissions and thus has attracted extensive attention recently. In this study, we propose a hybrid vehicle platoon system for the connected and automated vehicles (CAVs). First, a hybrid spacing policy combining the constant time gap (CTG) and constant spacing (CS) is formulated for the proposed platoon system, where the leader adopts the CTG and the followers use the CS policy. Based on the h2-norm string stability criteria, the notions of exogenous-head-to-tail string stability and hybrid string stability are newly introduced, and the sufficient conditions of the hybrid string stability in the frequency domain are derived using the Laplace transform. Numerical experiments are conducted to validate the hybrid string stability. Moreover, two typical scenarios and several measurements of effectiveness (MOE) are adopted to verify the effectiveness of the proposed hybrid platoon system in various aspects. The results show that the hybrid platoon system performs better than the CS-based platoon system. It also indicates that the hybrid platoon system has obvious advantages over the CTG-based platoon system under the periodical fluctuation scenario and it is also comparable to the CTG-based platoon system under the large deceleration and acceleration scenario. The findings have demonstrated the merits of the combined implementation of CTG and CS policy in enhancing the performance and applicability of the platoon system for CAVs.