Conventional RFID interrogators use symbol-by-symbol-based detectors such as the matched filter and the edge detector for signal detection which do not sufficiently exploit the structure of the data-encoded waveforms. In this paper we propose a Viterbi-based algorithm to detect the data sequence of the RFID systems using the EPC-Global Generation-2 standard. By decomposing the associated data-encoded waveform on a half-cycle basis, the proposed approach first transfer the representation of the data-encoding scheme to a trellis diagram and thus leads to the application of the Viterbi algorithm for data sequence estimation. Also, different from the traditional Viterbi algorithm using correlation coefficient as the branch metrics, the proposed algorithm uses the absolute correlation coefficient as our branch metric which can exactly save the computational complexity of the Viterbi algorithm up to 50 percent. As compared to conventional approaches, the proposed algorithm can significantly improve the system performance due to its full exploitation of the baseband signal structure.