Presently, the load of the shield main bearing cannot be directly characterized, and the load spectrum of evaluation tests is unavailable. Therefore, a load characterization method for the shield main bearing under complex operating conditions is proposed herein. Initially, the force analysis of the main drive with a telescopic swing mechanism is conducted, and a model is established. Subsequently, based on the main drive mechanical model, 22 parameters including pressure, displacement, and motor torque of the telescopic cylinder associated with the main drive of the telescopic swing are selected as analytical parameters, and the unknown quantity in the equation is determined based on the balance equation with the characteristic data and the driving attitude in the tunneling process. Thereafter, the engineering geological conditions of a metro tunnel in Dalian, China, are analyzed, and the construction characteristic data of the shield are extracted. Further, the load spectrum of the main bearing is compiled by statistical analysis methods such as rain flow counting, distribution estimation, and load extrapolation. Finally, the compiled load spectrum is utilized to verify the calculated life of the main bearing. The results show that the proposed method can accurately characterize the load level of the main bearing under certain engineering geological conditions and provide a load basis for the subsequent design and testing of main bearings.