A gourd-shaped contraction-expansion design is proposed for a passive planar micromixer in this study. The mixing performance of the micromixer is analyzed numerically and compared with a T-shaped planar micromixer. The gourd-shaped contraction-expansion structure can enhance the vortex-formation and mixing abilities of the micromixer. The numerical simulation reveals that the gourd-shaped structure can improved vortex generation and mixing efficiency within a high Reynolds number range. The micromixer with an optimized waist width of 50 μm reaches a mixing efficiency of approximately 83.25% and maintains a moderate pressure drop of 4860 Pa at Re = 100. This study can shed light on the design of new 2D micromixers from the point view of bionics.