Tag anti-collision algorithm is a significant technique in RFID system. This paper introduced its two wide used categories, stochastic anti-collision algorithm and deterministic anti-collision algorithm, and then proposed an improved algorithm using hash method. The proposed algorithm divides the tags into a number of groups using hash function and allows only the tags in group0 to respond. If there are more than one tag in group0, divide this group in to a number of subgroups. The hash method will avoid that those conflict tags were divided into the same subgroups. The simulation performance shows that the algorithm is effective especially in high-load situation.