This paper presents an effective approach to design a dual-band circularly polarized (CP) slot antenna with flexible frequency ratio (FFR) and similar in-band gain under the 0.5- and -mode radiation. Initially, a CP slot antenna is realized in virtue of two orthogonal slot radiators. Then, by folding both slot radiators at a 1/4 length position from the short-circuited terminal, a CP slot antenna operating at two distinct bands is successfully realized based on the 0.5- and -mode, where stable gain with similar level for the dual working bands is achieved. After that, two pairs of slot stubs are properly introduced to effectively reallocate the higher band based on the -mode, thus acquiring for a wide frequency ratio ranging from 1.85 to 1.26. Finally, for experimental validation purposes, two specific CP slot antennas with distinctive frequency ratios of 1.85 and 1.26 are implemented and tested. And a similar-level gain within their dual operating bands can be achieved as another merit in addition to the FFR.