The present invention relates to a low-cost terahertz frequency selective surface processing system and method. The system comprises a femtosecond laser, a diaphragm, an energy attenuation module, a shutter, a dichroscope, a CCD camera, a monocular lens, a mobile platform, a coaxial light source, an object lens and a computer. The laser emitted by the femtosecond laser is focused on a sample fixed on the mobile platform through the diaphragm, the energy attenuation module, the shutter, the dichroscope and the object lens in order to obtain a focusing light spot; in the processing process, the coaxial light source is used for luminescence to illuminate a processing position, the reflection light is irradiated on the CCD camera through the monocular lens for imaging and performing display on a computer so as to assist in regulation of the quality of the focusing light spot and observation of the processing quality in real time; the energy attenuation module comprises a [Lambda]/2 wave plate and a Glan prism, and the sample is an aluminum foil; and the shutter is configured to control the switching of the laser. The technology is simple, and the cost is low.