The paper considers thoses external environmental factors that will influence enterprise entry in micro~analytical frame in terms of enterprise~investment subject and researches the decision selection about enterprise entry under cluster effects by refering the real option pricing model. Furthermore how  the cluster effects influence the entry timing is discussed. This is benefit to understand the mechanism under which the cluster effects will influence investment in essence. The results indicated that the accumulating effect and policy effect lowers the critical value of enterprises entry and promotes enterprises entering; The congestion effect and uncertainty effect increase the threshold and delay enterprises entry. Different types of business have different reactions to agglomeration and crowding effect, therefore enterprises enter into the park at a different timing with the growing of clusters.