The concept and principle of optical fiber grating chirp-sensing technology are proposed. The spatially varying structure of the chirped fiber grating has been divided into several sections called the effective-interaction sub-regions. Each effective-interaction sub-region prossesses an independent sensing ability and its relationship to the gradient distribution of environmental parameters has been formulated. The origin of the FBG chirping and its effect on the characteristics of the resultant reflection spectra (wavelength, bandwidth and total optical power) under the effect of the surrounding environment (spatial distributions of temperature and strain) have been theoretically explained and experimentally demonstrated. This provides the foundation of the FBG chirp-sensing technology and opens new avenues for its potential applications in simultaneous measurement of multiple parameters and also in the distributed measurement of nonuniform environmental parameters.