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Understanding the development of cracks in masonry walls can provide insight into their capability for earthquake resistance. The crack development is characterized by the displacement difference of the adjacent positions on masonry walls. In seismic oscillation, the instantaneous dynamic displacements of multiple positions on masonry walls can warn of crack development and reflect the propagation of the seismic waves. For this reason, we proposed a monocular digital photography technique based on the PST-TBP (photographing scale transformation-time baseline parallax) method to monitor the instantaneous dynamic displacements of a masonry wall in seismic oscillation outdoors. The seismic oscillation was simulated by impacting a suspended steel plate with a hammer and by simulation software ANSYS (analysis system), for comparative analysis. The results show that it is feasible to use a hammer to impact a suspended steel plate to simulate the seismic oscillation as the stress concentration zones of the masonry wall model in ANSYS are consistent with the positions of destruction on the masonry wall, and that the crack development of the masonry wall in the X-direction could be characterized by a sinusoid-like curve, which is consistent with previous studies. The PST-TBP method can improve the measurement accuracy as it corrects the parallax errors caused by the change of intrinsic and extrinsic parameters of a digital camera. South of the test masonry wall, the measurement errors of the PST-TBP method were shown to be 0.83mm and 0.84mm in the X-and Z-directions, respectively, and in the west, the measurement errors in the X-and Z-directions were 0.49mm and 0.44mm, respectively. This study provides a technical basis for monitoring the crack development of the real masonry structures in seismic oscillation outdoors to assess their safety and has significant implications for improving the construction of masonry structures in earthquake-prone areas.