Up to now,many neural coding theories have been proposed,such as rate code,temporal code,population code,etc.However,many of them contradict with each other.We think that theoretical consideration is the basis to judge which proposal is more reasonable.Objective criteria are required for assessing neural coding theory.In the paper,four obligatory conditions are suggested.First of all,we give the hypothesis that humans neural system works in the ordered space,based on which the conclusion that neural coding takes the form of spike neural sequence can be drawn by computational analysis on the neuron model.Furthermore,the signal process for the neural network in olfactory system is investigated.The strength of various odors sensed by olfactory sensory neuron can be read directly from the output spike sequence of the mitral cell.