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Evanescent wave absorption-based mid-infrared chalcogenide fiber sensors have prominent advantages in multicomponent liquid and gas detection. In this work, a new approach of tapered-fiber geometry optimization was proposed, and the evanescent efficiency was also theoretically calculated to evaluate sensing performance. The influence of fiber geometry (waist radius (<i>R</i>_w), taper length (<i>L</i>_t), waist deformation) on the mode distribution, light transmittance (<i>T</i>), evanescent proportion (<i>T</i>_O) and evanescent efficiency (<i>τ</i>) is discussed. Remarkably, the calculated results show that the evanescent efficiency can be over 10% via optimizing the waist radius and taper length. Generally, a better sensing performance based on tapered fiber can be achieved if the proportion of the <i>LP</i>₁₁-like mode becomes higher or <i>R</i>_w becomes smaller. Furthermore, the radius of the waist boundary (<i>R</i>_L) was introduced to analyze the waist deformation. Mode proportion is almost unchanged as the <i>R</i>_L increases, while <i>τ</i> is halved. In addition, the larger the micro taper is, the easier the taper process is. Herein, a longer waist can be obtained, resulting in larger sensing area which increases sensitivity greatly.