The experimental research of fiber grating underwater pressure sensing with high sensitivity is reported. The sidehole package technique is presented,which is a novel fiber grating pressure sensitivity enhancing scheme. Its work principles and techniques are also studied. The practice of packaging with ploymer is done and the measurement results show that the pressure sensitivity of fiber grating with package is 5 251 pm/MPa,which is 1 750 times of that of without package. The side-hole package scheme reduces the dependence of pressure sensitivity on the material parameters, and it can satisfy the underwater pressure measurement requirement with high accuracy.