The context of this isolated film needs to be understood before leaping to judgment. The officer tried to arrest the woman before she evaded him and crossed over the on/off ramp out onto the freeway shoulder where the officer followed her. It is against California law with big fines to walk on a freeway. She again resisted and he subdued the woman (whether he used excess force is another issue). Then a plains clothes Highway Patrol officer suddenly appears and helps to subdue and arrest the woman. This suggests there was a possible prostitution stakeout of a person soliciting on a freeway ramp. And if you know anything about the world of prostitution the woman could have been a man. So now I have painted a plausible context for interpreting the video.
Most of the responses defending him on this thread for punching her in the face 11 or more times are sickening.