In this paper the important feature of artificial neural networks —— parallel distributed processing is discussed thoroughly，so the essential features of neural computing are revealed，and the important conception of timestep is introduced. At the same time，two critical concepts of mapping algorithm —— balancing the working load of PEs and the communicatin overhead among PEs are explained more thoroughly，and the main allocation criterions are presented. Adopting the ideas of graph theory，an optimal neural network mapping algorithm —— Absorb Algorithm is advanced. This mapping algorithm shows remarkable efficiency. Some interesting experimental results are presented.