A kind of surface plasmonic waveguide (SPW) with a double-square-shaped air core was designed. The propagation properties of the fundamental mode supported by this SPW are analyzed using the finite-difference frequency-domain method. The result shows that the longitudinal energy flux density of the fundamental mode distributes mainly in the central area of the double-square-shaped air core. Adjusting the sides of the two squares, the center to center distance of the two squares and the working wavelength, the field distribution range as well as the degree of the interaction of field with silver can be adjusted, then the effective index, propagation length and mode area of the fundamental mode can be adjusted. Compared with the SPW with a double-circle-shaped air core, it is found that the propagation properties of this kind of SPW are better than the latter within 730~800 nm working wavelength range. This double-square-shaped SPW can be applied to the field of photonic device integration and sensors.