Relay attacks pose a serious threat to the security of radio frequency identification systems. The adversary manipulates the communication by only relaying the verbatim messages between a reader and a tag in order to increase the communication distance between them, which breaks the implicit assumption that a tag is actually within a very short distance of a reader. The main countermeasure against relay attacks is the use of distance bounding protocols measuring the round-trip time between the reader and the tag. In 2005, Hancke and Kuhn proposed the first distance-bounding protocol dedicated to RFID system named HK. From then on, a number of relative schemes have been proposed subsequently in literature. In this paper, we design a distance-bounding protocol suited for computation limited RFID tags against relay attacks. We firstly review the existing RFID distance-bounding protocols. Weaknesses and advantages in these protocols are examined. In addition, we propose an attack model for designing and analyzing RFID distance-bounding protocols. Finally, we propose a novel distance bounding protocol based on HK protocol named HKM. It mixes the predefined challenge and the random challenge, and takes advantage of the wasting memory of HK protocol. Compared with the existing distance bounding protocols, HKM has good performance in both memory consuming and resistance to relay attacks.