Tā dǎ zhe císhàn de qíhào, shíjìshàng shì zài piàn qián.
Under the banner of charity, he was actually swindling money.
Tips
usage
旗号 most often appears in the pattern 打着...的旗号 (under the banner of / using ... as a pretext), usually with a negative connotation - someone claiming a noble reason while doing something else.
