Provided is a method of predicting fouling during a process of preparing polyolefin. According to the present invention, occurrence of fouling may be predicted by calculating R value according to the following Equation 1 in real-time with high reliability during a copolymerization process of preparing polyolefin: R (ratio of unreacted alpha-olefin comonomer to produced polyolefin polymer) =amount of unreacted alpha-olefin comonomer (unit: kg/hr) / amount of produced polyolefin polymer (unit: kg/hr)   [Equation 1] Therefore, productivity of the polyolefin preparation process may be further increased.