An experimental system was setup to study the pressure field of unconfined vapor cloud explocions. The semi-spherical vapor clouds were formed by slotted 0.02 mm polyethylene film. In the center of the cloud was an ignition electrode that met ISO6164 "Explosion Protection System" and NFPA68 " Guide for Venting of Deflagrations". A data-acquisition system, with dynamic responding time less than 0.001 s with 0.5% accuracy, recorded the presure-time diagram of acetylene-air mixture explosion with stoichiometrical ration. The initial cloud diameters varied from 60 cm to 300 cm. Based on the analysis of experimental data, the quantitative relationship is obtained for the cloud explosion pressure, the cloud radius and the distance from ignition point. Present results provide a useful way to evaluate the building damage caused by unconfined vapor cloud explosions and to determine the indispensable explosion grade in the application of multi-energy model.