To rapidly acquire target position information and achieve high-precision imaging detection of targets in the forward-downward-looking scenario of a flying platform, this paper proposes a real-time imaging algorithm for terahertz-band Synthetic Aperture Radar(SAR) applicable to dive scenarios, based on a forward-downward-looking three-dimensional imaging model under the uniform linear array mode. The algorithm simplifies signal processing through reasonable approximations to reduce computational complexity, thereby enabling rapid three-dimensional scene reconstruction. The feasibility of the algorithm is verified through a simulation platform, and the algorithm is mapped to a high-speed parallel Field-Programmable Gate Array(FPGA) hardware architecture. By comparing the FPGA processing results with the simulation results, the feasibility and real-time performance of the algorithm in an actual hardware system are further validated.