This paper analyzes how interest groups alter their contributions and proposal price of tariff when facing the political contribution caps. We study the change in the social welfare when setting political contribution caps are set. It is found that setting the political contribution caps can achive the goal of improving the social welfare in most situations. However, when the contributions of the group originally winning the bidding are lower, the social welfare could be worsening if the group originally losing the bidding wins the bidding.