Since Christmas about £400,000 has been reported stolen through scams.
Authorities believe this figure could be even higher because of under reporting.
Nigel Henaghen from Guernsey's financial investigation unit said islanders need to be "more sceptical".
As a former UK Police officer he believes Channel Islanders can be "too trusting" and are a "victim of their own downfall".
He said:  "If you put yourself in the fraudsters' shoes, if you start getting some good results in a particular location such as the Channel Islands you're going to go back there.
"Financial crime is certainly on the up. Over the last 12 months scammers have become more sophisticated and we need to raise our game and raise awareness."
Action Fraud, the national fraud reporting centre, showed scams went up by a third in the Channel Islands in the last financial year.
The most common type of fraud was through online shopping and auction, but Guernsey Police said it had seen "a huge increase" in unsolicited phone calls purporting to be from bank officials.
In Jersey a total of £100,000 was stolen from a few islanders over a four-day period through a series of telephone scams.
Insp Dave Burmingham, from Jersey Police, said it was "extremely challenging to quantify the scale of fraud, due to the quality and quantity of data available".
However, he was not aware Jersey residents were at a "heightened vulnerability".  He said: "Jersey experiences similar offences as Guernsey and the UK".
"The current theme coming to attention is advance fee fraud, often received by post. Typically these can be rental fraud, lottery prize, inheritance, and dating scams," he added.
Fraudsters are targeting people in the Channel Islands more than other jurisdictions because they are "too trusting", according to Guernsey Police.