The physical changes of tissue are complicated to evaluate during optical clearing (OC) treatment. Monitoring the changes of optical parameters, including the complex refractive index (CRI), helps people better understand the OC process. From the imaginary part of CRI, we can deduce the extinction coefficient of tissue. Based on the total internal reflection method, the time-dependent CRI of porcine muscle during natural dehydration is well determined. Results show that the real RI increases continuously with the increase of dehydration time, whereas the extinction coefficient initially increases and then decreases. Finally, the extinction coefficient becomes much smaller than the initial value, which demonstrates that better tissue optical clarity is obtained. The change tendency of the extinction coefficient of tissue is used to qualitatively explain the dynamic change of transmittance of a natural dehydrated tissue. Consequently, CRI, especially its imaginary part, is a very useful optical parameter by which to evaluate the OC effect.