1. An on-line identifying method of hand-written Arabic letters, comprising steps of: (a) collecting chirography coordinates of hand-written Arabic letters that are inputted in a terminal unit, and storing said chirography coordinates into a predefined structural array in real time; (b) preprocessing chirography coordinates and chirography coordinates lattice in a matrix format transformed from said chirography coordinates stored in a structural array; (c) by using multilayer coarse classification algorithm based on local characteristic of Arabic letter, according to shape characteristic of Arabic letters, classifying all standard Arabic letters into four categories, obtaining a first candidate letter aggregation matching with inputted hand-written Arabic letter according to stroke number of inputted hand-written Arabic letter, and obtaining a second candidate letter aggregation matching with inputted hand-written Arabic letter according to local characteristic of inputted hand-written Arabic letter and first candidate letter aggregation; and (d) extracting a freeman chain code of inputted hand-written Arabic letter, calculating a matching probability between said freeman chain code of inputted hand-written Arabic letter and an optimal Hidden Markov Models of each standard letter stored in a predetermined letter library and corresponding to each letter stored in the second candidate letter aggregation, obtaining an optimized matching probability from each matching probability, and determining that standard letter stored in a predetermined letter library and corresponding to an optimized matching probability as final identifying result of inputted hand-written Arabic letter.