Two parallel line waveguides were formed by removing two lines silicon pillars which alternate with three line pillars in photonic crystal. Its coupling properties were investigated by use of Finite-difference Time-domain (FDTD) method. It is found that the coupling length between two waveguides can be changed significantly by turning the size of silicon pillar and background refractive index. A new liquid refractive index detection method was brought forward based on this research. Compared with normal liquid refractive index analyzer reported, the most important aspects are the fast detection and miniaturization of devices.