The industry of Small Loan Companies ( SLCs) is developing very fast in China，but the problem of financing difficulties of Small and Micro Businesses is still serious because of Mission Drift in SLCs． Most studies examine Mission Drift from the perspective of financial sustainability; this paper intends to investigate it from the perspective of Ｒelationship Lending and Market Structure． By building a theoretical model，it finds that relationship lending and a competitive market structure are desirable，if SLCs are to supply more loans to satisfy the demand of Small and Micro Businesses and avoid Mission Drift． Furthermore，the paper analyzes SLCs in a representative municipal area of China，and believes that the reason of Mission Drift is insufficient use of the technology of relationship lending by SLCs，and the highly monopolized microcredit market．