Abstract: A one-dimensional optomechanical crystal nanobeam was designed and fabricated using typical CMOS process. The optical mode (in the fiber optic communication band) and the acoustic mode (~5.344 GHz) were characterized at very low temperatures (28 mK), and the phonon counting experiment was achieved using pulsed optical pumping and single photon detection methods. This kind of pulsed optical pumping can scatter photons at the cavity frequency and can generate (or reduce) acoustic phonon numbers. This pulsed light pumping method can also reduce the heating effect of light and maintain low phonon occupancy; then the pump photons are selectively filtered by cascading narrow-bandwidth fiber Fabry-Pérot filters, after which the scattered photons are measured to accurately calculate the average phonon occupancy of the mechanical mode. The experimentally obtained average phonon occupancy is 0.14 \pm 0.03 , reaching the few phonon level, which lays the foundation for application of acoustic quantum state in quantum computing, quantum precision measurements, quantum transducer and other related fields.