The stopped-flow method, by which a quasi-living polymerization process can be realized within an extremely short period (ca. 0.2 s). has been proven to be one of the most powerful techniques for studies on the nature of active sites and elucidation of the olefin polymerization mechanism based on the information from the polymers obtained in the initial stage of polymerization. It has been demonstrated that a better and deeper understanding many controversial problems in Ziegler catalysts has been achieved, such as arguments concerning the non-uniformity of the active sites, the intrinsic kinetic parameters the effects of catalyst preparation conditions, the role of cocatalysts, hydrogen and electron donor, etc. The successive modifications to the basic stopped-flow system have led extensive applications in investigating the catalyst pretreatment effects induced by various reagents, as well as developing a series of novel blocks copolmers.