As another party other than management and shareholders, institutional investors have been highly expected from scholars at home and abroad and government regulators to improve the level of corporate governance. However, existing studies on the institutional investors' governance roles have formulated three arguments in corporate governance, including valid supervision, invalid supervision and strategic conspiracy. In order to answer what roles institutional investor would play in corporate governance, this study takes two types of agency costs as the starting point, re-examines the governance behavior of institutional investors from the perspective of portfolio, and explains the contingency viewpoint of their participation in governance behavior under the different internal and external situations. Based on principal-agent theory and limited attention theory, this article takes A-listed publicly firms in Shanghai and Shenzhen markets over the period from 2013 to 2017 as example. By defining motivated monitoring institutional investors, from the two dimensions of portfolio weight and portfolio concentration, we explore the impact of institutional investors on the two types of agency costs by applying the fixed effect regression model. The empirical results are robust to alternative IV approach. In addition, combined with different situations inside and outside the company, we further dialectically analyze the differences of governance behavior of institutional investors with the help of the sub-sample regressions. The results display that:as a mode of investment behavior to spread risk, investment portfolio have double-edged sword effects for institutional investors in corporate governance. It means that monitoring institutional investors and portfolio concentration reduce the agency problems between large shareholders and minority shareholders, but improve principal-agency cost. Furthermore, the institutional investor's governance role is distinct considering external market environment, internal executives and controlling shareholders. To be more specific, the phenomenon, which the principal-agency cost is intensified by institutional investors who are in companies with underdeveloped regions or powerful executives, is more obvious. While the restraining effect of institutional investors on the agency problems will be strengthened when supportive controlling shareholders exist in the company. Using the view of contingency, the above-mentioned research has realized an creatively change from the focusing on the "number of shares" of institutional investors to "portfolio concentration". It not only enriches research perspective of institutional investors, but also supports the rationality of the governance behavior of institutional investors from contingency view. Our findings can help firm to carry out management innovation by introducing motivated monitoring institutional investors, which improve its governance level to achieve high-quality development. At the same time, conclusions could offer reference for CSRC to innovate the regulatory model, fully understand the governance behavior of institutional investors, and formulate relevant documents to guide heterogeneous institutional investors.