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A kind of surface plasmonic waveguide (SPW) with a double-petal-shaped air core is designed. The propagation properties of the fundamental mode supported by this waveguide on geometrical parameters and working wavelengths are analyzed by using the finite-difference frequency-domain method. Results show that the longitudinal energy flux density is mainly distributed in the two wedged corners formed by the air core. The propagation length, effective index and mode area of the fundamental mode can be adjusted by the shape of petal-type air cores. With certain geometric parameters, relative to the case of 632.8 nm, the propagation length becomes large under the condition of larger working wavelength. A comparison of SPW proposed here and SPW with double elliptical air cores has been carried out, and results show that the SPW with a double-petal-shaped air core not only has no sharp tips, but also has better propagation properties than that of double-elliptical air cores under certain conditions. When the core is filled with gain medium, it is found that the propagation length can be extended obviously with the help of the gain dielectric medium. This kind of SPW can be applied to the field of nanoscale circuits and sensors.