In this paper, we study relay selection under outdated channel state information(CSI) in a decode-and-forward(DF) cooperative system. Unlike previous researches on cooperative communication under outdated CSI, we consider that the channel varies continuously over time, i.e., the channel not only changes between relay selection and data transmission but also changes during data transmission. Thus the level of accuracy of the CSI used in relay selection degrades with data transmission. We first evaluate the packet error rate(PER) of the cooperative system under continuous time-varying fading channel, and find that the PER performance deteriorates more seriously under continuous time-varying fading channel than when the channel is assumed to be constant during data transmission. Then, we propose a repeated relay selection(RRS) strategy to improve the PER performance, in which the forwarded data is divided into multiple segments and relay is reselected before the transmission of each segment based on the updated CSI. Finally, we propose a combined relay selection(CRS) strategy which takes advantage of three different relay selection strategies to further mitigate the impact of outdated CSI. In this paper, we study relay selection under outdated channel state information (CSI) in a decode-and-forward (DF) cooperative system. Both previous researches on cooperative communication under outdated CSI, we consider that the channel varies continuously over time, ie Thus the level of accuracy of the CSI used in relay selection degrades with data transmission. We first evaluate the packet error rate (PER) of the cooperative system under continuous time-varying fading channel, and find that the PER performance deteriorates more seriously under continuous time-varying fading channel than when the channel is assumed to be constant during data transmission. improve the PER performance, in which the forwarded data is divided into multiple segments and relay is reselected before the transmission of each segme nt based on the updated CSI. Finally, we propose a combined relay selection (CRS) strategy which takes advantage of three different relay selection strategies to further mitigate the impact of outdated CSI.