An improved semantic distance for short text is proposed. The new method calculates the semantic distance between two word strings as balance of the extent of word sequence alignment and the meaning matching between word strings. First, after linguistic preprocessing, the extent of word sequence alignment is computed by the structural distance which measures the maximum matching based on the HIT-CIR Tongyici Cilin (extended edition). Then the meaning matching between word strings is computed by an improved edit distance which allocates each word a weight according to its word type. Finally, the semantic distance between the word strings is measured as a balance of structural distance and word meaning matching distance. In addition, in order to eliminate the influence of the sentence length, the proposed semantic distance is adjusted using the distinct word count estimated by the Heap's law and Zipf law. Experimental results show that the presented methods are more efficient than the classical edit distance models.