Due to the advantages of high resolution, low cost, small size, easy deployment and capability of multiplexed sensing, the recent developed optical fiber grating sensors provide powerful tools for crustal deformation monitoring. This paper reviews the development of several types of fiber-optic sensors with ultrahigh resolution in quasi-static domain. The fiber Bragg grating based Fabry-Perot interferometers and the -phase-shifted fiber Bragg gratings which are used as sensing components in the high resolution sensors are introduced. Some novel techniques such as interrogating the sensing components with intensity modulation sideband, dual feedback-loop structure for high bandwidth/large measurement range sensing, and the time-domain multiplexing of the high resolution quasi-static strain sensor are discussed in detail. Each sensing scheme with both operation process and achieved performances are given. The implementation of fiber grating sensors for in-situ measurement of crustal deformation and the results are also introduced. Compared with the traditional methods used in crustal deformation observation, the high-performance fiber optic strain sensors mentioned in the paper shows great potentials in providing wider measurement approaches in geophysical researches.