Multi-temporal interferometric synthetic aperture radar (MT-InSAR), as one of the geodetic techniques, is widely used in geological disaster monitoring and engineering applications. However, modern satellites provide an endless array of SAR imagery for InSAR applications, which has prompted us in exploring more efficient and accurate MT-InSAR methods to monitor the ground stability continuously and early detect the ground hazards. We proposed in this paper a step-by-step estimator for continuous deformation monitoring, aiming to study the ground surface deformation that has occurred in the past few years and monitor the latest developments in the same location continuously. Associated with the existing results of MT-InSAR, when a new SAR image is available, the unwrapped phase vector is sequentially estimated with the ambiguity detection, and then the deformation parameters are updated timely. Through experimental verification over Hong Kong, the proposed estimator shown its capability to detect the abnormal changes and update the deformation sequentially without being affected by the complex deformation histories.