Thermodynamical stability boundaries of ethylene hydrate system were investigated in a temperature range from 279 to 328 K and pressure range up to 465 MPa. The three-phase coexisting curve of the ethylene hydrate + saturated water + saturated fluid ethylene shows the characteristic "S-shape" behavior in the pressure-temperature projection. From the fugacity-temperature projection, it can be concluded that such curious behavior is caused by the anomalous critical phenomenon. The laser Raman microprobe spectroscopic analysis reveals that the stretching and bending vibration energies of ethylene entrapped in the hydrate cage, fulfill two different conditions. It has been argued that ethylene occupies only the large cage of structure-I. However, based on the double peaks from the Raman scattering on ethylene hydrate crystals, we give direct evidence that, in spite of ethylene's large van der Waals diameter, it also occupies the small cage.