Competitive bidding system, bearing the advantages of simplicity and fairness, is expected to encourage economic benefits through the free market competition mechanism. However, as the characteristics of construction industry are different from those of other industries, many issues such as abnormal low-bids and poor project quality have been derived from the competitive bidding system, and hinder the development of the construction market. By using System Dynamics this research developed a contractor’s pricing model consisting of two intended economic feedback structures with competitive bidding philosophy and an unexpected adverse one, namely Opportunistic Bidding feedback structure. The results suggest that the ideal competitive bidding system is only effective when contractor’s opportunistic bidding behaviors are restrained. The emphasis of government’s policies should be placed on inhibiting the Opportunistic Bidding, as it is the leverage point to improve the efficiency of public construction market.