Utilizing some parameters of national ignition facility (NIF), the two-lens slit spatial filter (SSF) replaces the pinhole spatial filter in main amplifier cavity for the high-power laser facility, and the application is studied preliminarily. Take NIF as an example, the far-field intensity decreases two orders of magnitude due to the SSF, reducing the cavity spatial filter and the amplifier cavity by about 6.2450 m (27%) and 13.8707 m (30%) respectively. Basically there will be no pinhole (slit) closure problem in the main optical system since the reduction of far-field intensity and the absences of filter aperture in transport spatial filter. Additionally, based on the design of off-axis angle complanation combined with the characteristics of mutual imaging between the distorted mirror and the reflector in amplifier cavity, the off-axis multi-pass of cavity spatial filter composed with SSF is designed to avoid the amplified spontaneous emission, which lays the foundation for the practical application of SSF and the further development of high power lasers.