The sensitivity of an interferometer based on intermodal interference in photonic crystal fiber (PCF) has been investigated theoretically and numerically. Using the intermodal interference theory and the normalized frequency of PCF, we have analyzed the principle of the method for measuring refractive index by PCF interferomter,and obtained a variation, δns, to show the sensitivity of the PCF interferometer. From the analysis of the underlying physics of δns, we find that air holes of the first layer play an important role on the sensitivity.Compared with the conventional PCF structure,the sensitivity can be improved remarkably via employing large air holes with the shape of a combination of half circle and half oval.