Vehicular ad-hoc networks (VANETs) are one of the most important components in an Intelligent Transportation System (ITS), which aims to provide information communication between vehicles. A safety-critical vehicular communication requires security, privacy, auditability, and efficiency. To satisfy these requirements simultaneously, several conditional privacy-preserving authentication schemes are proposed by employing ring signature. However, these methods have been paid too little attention to the issues like \textit{how to choose the valid ring members} or \textit{how to set up a ring}. In this paper, we introduce an efficient conditional privacy-preserving scheme which provides an appropriate approach establishing the list of ring members. Moreover, our proposed scheme also supports batch verification to significantly reduce the computational cost. According to the analysis of security, our scheme is sufficiently resistant against several common attacks in VANETs. The performance results show that the proposed scheme is efficient and practical with both low computation and communication cost.