The penetration depth of evanescent wave, sensing diameter and sensing length in the uniform sensing region were the main influence factors of sensitivity for the fiber based evanescent wave sensors. To optimize the sensitivity design, the mathematical models of the relationship between the penetration depth of evanescent wave and the parameters of linear taper geometry as well as the relationship between the effective absorption path and the sensing diameter and sensing length were established respectively. The evanescent wave penetration depth in different taper parameters (i.e. emission angle, taper ratio and taper length) and the effective absorption path in different sensing diameter and sensing length were discussed and simulated. The result of simulation showed that the penetration depth could be increased by a factor of 3 through choosing the tapered fiber geometry and appropriate emission angle, and the penetration depth could be achieved maximum when the taper ratio reached to 0.4. Moreover, reducing sensing diameter and increasing sensing length could enlarge effective absorption path, thus enhanced the interaction strength between the evanescent wave of fiber core surface and the surrounding absorbing medium. The research result could be used to fabricate the highly sensitive fiber based evanescent wave sensor.