谁是卧底海报怎么说英语

"Who is the undercover agent?" is how you would say "谁是卧底" in English. "海报" is translated as "poster" in English. So the full phrase "卧底海报" would be translated as "undercover agent poster." It's not a very common phrase in English, so it's possible that native English speakers may not understand it. If you are trying to convey the idea of a poster that features an undercover agent or that relates to undercover agents, you could say "a poster featuring an undercover agent" or "a poster about undercover agents."