Cancer is one of the leading causes of mortality worldwide. In recent years, various kinds of biosensors based on optical fiber have been proposed for detection of cancer cells due to their advantages of accurate diagnosis, small size, low cost, and flexible design parameters. In the present study, a microstructure fiber (MSF) biosensor with porous-core structures was designed to detect cancer cells using a terahertz time-domain system (TDS). The fiber characteristics of the proposed MSF were optimized by adopting a finite element numerical technique and perfectly matching layer absorption boundary conditions. The numerical results show that the proposed biosensor presented an ultrahigh sensitivity for detection of cancer cells. Under the optimal condition of 0.9 THz, the relative sensitivity of the proposed structure to breast cancer cells was as high as 99.8%. Moreover, other optical fiber parameters, such as effective material loss (EML), confinement loss (CL), numerical aperture (NA), power fraction, and effective area (A(eff)), were optimal according to the reported results. The proposed structure can be easily fabricated by 3D printing and flexibly applied in the fields of biomedicine and biosensing with a terahertz (THz) waveguide.