Abstract: We propose a compact photonic crystal Mach-Zehnder interferometer index sensor with a two-dimensional rotated square lattice, based on the photonic crystal self-collimation effect. Using the same background index, the different index of the circular rods has different effective index. The line defected and the air slab waveguide are respectively used as the beam splitter and mirror of the interferometer, and a sensing area is setting on one arm of the interferometer. The index of the circular rods changes with the concentration of the solution filled into the sensing region, thereby affecting the center wavelength of the transmission spectrum, and establishing a mathematical relationship between the solution concentration and the transmission wavelength. Then we carry out a numerical simulation of the concentration measurement of the alcohol solution, the results show that the sensor sensitivity is 250 nm/RIU in the refractive index variation within the scope of 1.33-1.37.