This paper designs and implements a Tibetan word segmentation system named "SegT".It identifies critical words with a fast algorithm based on the trie structure when it segments each Tibetan sentence to blocks with case-auxiliary words.Then,it identifies abbreviated words when it segments each block to words by maximum matching.Finally,it detects ambiguities by bidirectional segmentation,and solve them by word frequency.Experiments show that it improves the segmenting speed by about 15% after applying the block segmentation method based on case-auxiliary words,but the block segmentation doesnt significantly increase or decrease the precision.The precision of the system reaches 96.98%,which shows that its a practical system.