Detailed derivation is conducted to solve the Bloch wave solutions of the electromagnetic wave theory of two-dimensional photonic crystal and periodic media. The theoretical basis for the existence of the band gap in the photonic crystal is given. We measured the concentration of fructose solution in a two-dimensional triangular lattice photonic crystal using plane-wave expansion method. To get the numerical simulation of photonic band diagram for analysis, we take advantage of the large permittivity of silicon and fructose solution. The results show that the photonic band gap (PBG) of TE mode distinctly broadens with the difference of the concentration of the fructose solution but there is no PBG in TM mode, when fructose solution is used as the dielectric material in the air hole. The results of this study and the proposed method may play a guiding role in clinical applications for helping hyperglycemia patients.