Prince Andrew is due to arrive today in the Gulf state, which violently suppressed pro-democracy protesters in the Arab Spring of 2011.
A spokeswoman for the charity said: "Anyone who visits Bahrain should go there with their eyes wide open about the human rights abuses being committed by the police, sanctioned by the government.
"They have long ago reneged on promises to reform and the country is now trapped in an endless circuit of protest-clampdown-further protest-further clampdown. Over recent years we've had police officers acquitted of murder and torture charges or given disgracefully lenient sentences, and protesters - including children - given very long prison sentences."
The charity last month published a report which accused Bahrain of routinely arresting and torturing children as young as 13.
The UK Government requested or Prince Andrew to visit Bahrain.
The country's government, dominated by members of its royal family, promised reforms to meet the recommendations of an independent inquiry into the 2011 disturbances. Critics say little has changed since.
Prince Andrew's visit, expected to last a few days, coincides with a week-long Great British campaign in Bahrain.