The research on microstructure pattern and macroscopic formability of the ZA27 alloy worm wheel blanks by semi-solid forging experiment under different specific pressures and mold temperatures was stated. The results show that under less specific pressure, the worm wheel can not be formed, while the specific pressure is 4060MPa,its formability is good, and when the specific pressure is too high, it is easy to form the flash and it is hard to demould.With the increasing of the specific pressure, the microstructure of the parts changes obviously, and the spherical and rose-shaped semi-solid microstructure will break and deform as well as arrange in tortoise porphyritic. When the mold temperature is lower, the semi-solid slurry is difficult to fill the mould, and when the mold temperature is 200℃,its formability is well. Under different mold temperatures, there is a great difference in the microstructure of addendum of the worm wheel, it changes from the rose-shaped semi-solid microstructure to the broken microstructure.