The Blues face the reigning champions on Wednesday evening for the opening leg of their semi-final at Stamford Bridge.
Despite acknowledging the quality of the Barcelona squad, England international Cahill has insisted that Chelsea have a fighting chance of progressing through to the final.
"They're beatable, for sure, every team is beatable. But the way they play football is phenomenal," said Cahill.
"I saw them against AC Milan, they scored three but could have probably had six against a fantastic team in their own right.
"We're under no illusions and everyone knows it will be one of the toughest games of the season."
Cahill joined Chelsea in January from Bolton Wanderers.