There is obvious network externality in internet information service industry, but the flaw of current inter-share mechanism causes the unfair competition acting frequently, which hindered the healthy development of the market. The principal-agency relationship was studied between goverment and enterprises. A parameter was introduced to the Holmstrom-Milgrom model which represent the effciency of the internet information services enterprises implementing inter-share;under imperfect information, the variation of certainty equivalent income and total agent cost were analyzed by using different inter-share efficiency possess, A simulation was proceded to the Holmstrom-Milgrom model based on Matlab software. The results show that, under imperfect information, the higher effort enterprises do,the less certainty equivalent income they get; facing the risk ,the total agent cost of enterprises with higher efficency is higher. Based on the conclusion above, the relevant policy suggestions were given.