The register 0X0304 of AD9557 controls DCO output level, which can be 3-level or 4-level, I hope to know this level means what? Can not find a description in the Datasheet.
This bit controls how the sigma-delta modulator performs its frequency division, and is not intended to be changed by the user. The frequency planning wizard takes care of setting this for you, and the only reason that this bit is documented is that the wizard may set this bit.
Can I ask why you're interested in knowing? While I like to answer users' questions about how our part works, I'm also hesitant to do so. In my experience, when users start asking questions like this, it leads to experimentation with changing bits such as this one, and we only seem to end up with more questions unrelated to using the part.
That said, I hope I've answered your question. I invite you to read up on sigma-delta modulator design, and in doing so,hopefully there will be a good discussion about 3-level versus 4-level output sequences.