This study focuses on the design and simulation of the transmission far infrared microscopic imaging optical system matched with the cooled staring focal plane array detector that operates in the wavelength range of 50—70 μm and contains pixel number 64×64, each pixel is 120 μm×120 μm in size in order to make the far infrared detector obtain optoelectronic signals with a high signal-to-noise ratio. Stray light analysis predicts the negative effect caused by cold reflection, and a solution is proposed to address it. The simulation results show that when the designed optical system’s spatial resolution, numerical aperture, focal length, effective magnification, and central wavelength are 200 μm, 0.25, 14 mm, 10, and 61 μm, respectively, the modulation transfer function value reaches 0.305 at the characteristic frequency of 5 lp·mm^-1, and the energy concentration of the surrounding circle of the system exceeds 80%. Meanwhile, the optical system’s produced object picture is easily distinguishable, indicating that it meets the functioning criteria of the focal plane array detector.