In-orbit sequence image Rendering Observation Technology（IROT） in space light field is proposed for a set of small celestial bodies. It integrates the surface reflection characteristics， orbital characteristics， and observation needs of small celestial bodies； provides parameters such as position sequence， light angle， and camera characteristics of the target at the observation time； and generates a physical rendering image set that conforms to the imaging characteristics of the detector. First， the methods of 3D reference model establishment， reflection characteristic file generation， physically based rendering， fast conversion method of the normalized spectrum， and gray response are studied. Then， by rendering the image authenticity test and terrain 3D modeling test， the closed-loop simulation observation verification is carried out for the images of Itokawa at a specific observation time and location. Experimental results indicate that by matching the two-dimensional projected topographic map of the reconstruction model， the root mean square of the image grayscale value registration is 0.178 5， corresponding to an elevation difference of approximately 0.062 8 m of the surface of the small celestial body. The rendering observation technology can effectively serve the reconstruction of the terrain model of small celestial bodies and the construction of a terrain relative navigation closed-loop analysis environment. In addition， to meet the needs of on-orbit fast calculation， the fast conversion method of the normalized spectrum and gray response is proposed. The calculation time is significantly reduced from several hours to seconds， while retaining the same accuracy.