The enhancement effect and mechanism of the micro-nano composite structure composed of micro-pyramid array and nano-scale alumina(Al2O3) film on the modulation performance of silicon-based optically controlled terahertz modulators is studied. Experiments show that the micro pyramid array on the silicon surface can significantly reduce laser reflectivity compared with semiconductor silicon wafers, improve laser utilization, and increase the area of terahertz control. More importantly, the nano-thickness Al2O3 film deposited on the pyramid array can further reduce the laser reflectivity and significantly improve the modulation effect of the terahertz wave. Under the laser power density of 95.5 mW/mm2, its modulation depth can reach up to 91.2%. The optically controlled terahertz modulator has high modulation depth at low laser power, and shows a huge application potential in the field of terahertz imaging and communication.