The spatially modulated snapshot imaging polarimeter (SMSIP) can measure two-dimensional state of polarization through using the spatial carrier to encode the Stokes vectors in a single image. The traditional reference calibration is not suitable for dynamic environment (such as temperature change), and the dynamic calibration of linear shear spatial modulation snapshot imaging is proposed in this paper. In the dynamic calibration used are two modified Savart polariscopes with shear along the same direction as the core modulation device. In addition, the thickness ratio of the two modified Savart polariscopes is 1∶2. The spatial modulation phase factor generated by the core modulation device can be deduced from the interferogram and thickness ratio so as to demodulate all the polarization information of the target. The significant advantage of the dynamic calibration is that the target detection and the system calibration are carried out simultaneously. The reference target is not required in the process. In this work, the detailed theoretical analysis of the dynamic calibration is presented. In addition, a laboratory experiment and numerical simulation are conducted to demonstrate the validity of the proposed dynamic calibration. The present study provides a new idea for calibrating the spatially modulated snapshot imaging polarimeter, and effectively promotes its applications in engineering under dynamic environment.