Optical tweezer has become a powerful and flexible tool for trapping and manipulating the micro-nano particles through a gradient force well formed by a highly focused laser beam, and it has a wide applications in the fields of biology, physics, chemistry, and medicine. Based on the 4π focusing system, the tight focusing characteristics of the radially polarized Gaussian beam and its radiation force to the metal particles are theoretically studied and compared with the results of traditional single-lens focusing system. Furthermore, the influence of the off-focus distance and the off-axis distance on the trap stiffness is also investigated in detail. Numerical results show that a focal spot with three-dimensional and spherical structure can be obtained in the 4π focusing system via the suitable parameters. This spherical focal spot can largely enhance the transverse and longitudinal trapping forces, and consequently enhance the trapping stability of metal particles of optical tweezer system.