Abstract: Porous silicon thin films were prepared by chemical etching method on monocrystalline silicon surface. P-type morphology of these films exhibited three tapes: crater, quasi nano-pillar-array and porous. Terahertz time domain spectroscopy of these samples from 0.2-10 THz showed that transmitted intensity of these samples was very different. Taking air as the reference, transmission intensity of crater sample from 0.2 to 6 THz dropped by about one half and intensity of other two samples only decreased about by one fifth. For filter characteristics, samples of crater one and quasi nano-pillar-array one had low-pass characteristics and porous sample had cascade band-pass characteristic. Cut-off frequency of quasi nano-pillar-array sample improved 3 THz than others. Furthermore, there were much absorption peaks in samples and positions of these peaks had very closed relationship to the micro-surface-structures of porous silicon films. Experiments showed that, both shape and size of these films appeared to change and control transmission properties of monocrystalline silicon in terahertz such as transmitted intensity, absorption frequency and cut-off frequency. So porous silicon can be a new wide spectral detecting and modulation materials from terahertz to visible light.