Abstract: Based on the IPO inquiry system, the paper introduces the collusion behavior to the inquiry system and sets up a principal-agent model of three layers which include the regulator, quasi-listed companies and the sponsors, and studies the characteristic of optimal collusion-proof behavior, pointing out keys which prevent the collusion behavior: strengthening the supervision of social media to the IPO inquiry process, increasing the collusion behavior penalties and improving the deterrence to insider trading and price manipulation. The paper also analyses the collusion behavior in reality, which include the following points: making full use of the supervision of the social media to collusion behavior, there is a close relationship between the company′s transparency and the collusion behavior,and there is no an effective incentive constraint mechanism between the underwriters and the sponsors.

Key words: inquiry system, collusion behavior, collusion-proof contract