A method and a system for reading radio frequency identification (RFID) tags are provided. The method includes the following steps. First, receive a tag signal from the RFID tag. Recover a data clock rate from the tag signal according to statistics of pulse lengths of the tag signal. Next, determine a frame synchronization point of a data frame following a preamble in the tag signal by a signal correlation between the preamble and a predetermined signal pattern according to the data clock rate. Finally, decode the data frame by using an adaptive Viterbi algorithm on an extended trellis diagram. The extended trellis diagram includes a plurality of nodes and a plurality of branches connecting the nodes. The nodes and the branches are arranged according to the modulation scheme of the data frame and possible variations of the data clock rate.