The 270° beam deflection system with magnets is widely used in medical electron linear accelerators and high-energy electron industrial irradiation accelerators. In this paper, three common structures of 270° deflection magnet are systematically analyzed and discussed. Numerical calculation and simulation methods are used for the two-magnet asymmetric deflection structure, the three 90° magnets deflection structure and the 70°+130°+70° deflection structure. The achromatic transmission conditions of the deflection systems are given, and the changes of beam envelope in the deflection and exit beam line are analyzed. After analysis and comparison, the advantages and disadvantages of the three structures are listed in detail, which has certain guiding significance for the selection of structures in specific application fields. The two-magnet asymmetric deflection structure is suitable for medical accelerators. The three 90° magnets deflection structure is suitable for irradiation accelerators that require long-distance drift at the exit. The 70°+130°+70° deflection structure can satisfy the non-destructive drift of a certain distance from the exit, and it achieves relatively low cost, therefore, it which is a more economical and suitable choice for industrial irradiation accelerators.