We propose a method for measuring instantaneous structural intensity (SI) in flat structures using finite difference approximations. Based on flat plate theory, stress components associated with bending moments, torsional moments, and shear forces are described in terms of vibration displacement. The derivation formulation is established by applying finite difference approximations to partial differential terms up to fourth order in both time and space. To achieve high temporal and spatial resolution, vibration displacement is measured non-invasively using a Laser Doppler Vibrometer (LDV). This approach ensures precision and avoids contact-induced measurement errors. The proposed method demonstrated strong correlation with finite element method (FEM) simulations, confirming its validity and practicality for analyzing instantaneous SI. This study highlights the potential of the proposed method for high-precision vibration energy analysis in structural applications.