"平版机上浆纸" can be translated as "dampening paper for offset press" or more specifically, if referring to the paper that has been pre-treated or coated for use on an offset printing machine, it could also be expressed as "offset printing dampening paper" or simply "pre-dampened paper for offset printing".
However, in the printing industry, the term might be more commonly referred to in a context-specific way, such as "paper for offset lithography with dampening" or simply "offset paper with a dampening layer" (if it's a special type of paper with a pre-applied dampening coating).
For a general and straightforward translation, "dampening paper for offset press" is a good choice, as it clearly indicates the paper's purpose and the type of printing machine it's intended for.