This paper establishes assembly model according to the intuitionistic graphics mode characteristics of petri net, and gets feasible assembly sequence according to the principle of petri net. Most of factors influencing assembly sequence are certainly qualitative, fuzzy, non-numerical, assembly sequence is regarded as a gray system, and grey clustering decision method is adopted to evaluate feasible assembly sequence. This paper analyzes the gray classification of the influence factor and studies grey clustering decision method steps. The analysis of example indicates this method can evaluate correctly the feasible assembly sequences according to the principle of petri net and obtain decision vector.