Using HOMO Field Ionization Model, the tunneling probabilities and the theoretical threshold intensities of the field ionizations of acetylene, ethylene, and ethane in intense laser field are calculated. C2H2, C2H4, and C2H6 were irradiated by 800 nm, 100 fs laser pulses with the intensity range of 1013-1014 W/cm2. A TOF-mass spectrometer was coupled to the laser system and used to experimentally investigate the field ionization of these molecules. The experimental ionization threshold intensities are obtained. The calculating results of the three molecules agree well with the experimental results, indicating that HOMO Field Ionization Model is valid for the ionization of polyatomic molecules in intense laser field.