This article proposes a double D-shaped dual-core biosensor based on photonic crystal fiber (PCF) and broadly studied for finding of composition of blood such as white blood cells (WBCs), hemoglobin (HB), red blood cells (RBCs), plasma, and water. Finite element method (FEM) and commercially available COMSOL multiphysics software are used for simulation and quantitative measurement for the proposed PCF biosensor. In this biosensor, two independent waveguide formed due to middle circular hole because of two solid cores. Every composition of blood has different refractive index (RI), which is key parameter for sensing performance of biosensor. In this PCF, middle small circular hole behaves as a sensing channel on which different component of blood is placed. As a result, it is observed that there is a shift in peak wavelength because of different RI of blood samples in the transmission curve due to the mode coupling theory. The maximum wavelength sensitivity calculated in x-polarized and y-polarized is 6500 nm/RIU and 7500 nm/RIU for RBC-HB, respectively. The maximum allowable detection limit is 0.02. The suggested PCF biosensor will therefore study the wide range of medical diagnostics. Therefore, it is concluded that the suggested biosensor will perform perfectly when it comes to detecting blood component.