Abstract: Underground pyrolysis of tar-rich coal is a green and low-carbon technology that effectively extracts oil and gas resources from coal. It represents the forefront of clean and low-carbon utilization of coal. The nano-pores present in coal play a vital role in the adsorption and desorption of tar and gas during the underground pyrolysis process. Therefore, understanding the evolution of nano-pore structure under underground pyrolysis conditions is a key scientific issue for enhancing the oil and gas yield of tar-rich coal through this process. To address this, the China Spallation Neutron Source (CSNS) - Small-Angle Neutron Scattering (SANS) technique was employed to simulate the in-situ pyrolysis environment of coal seams. This allows for the quantitative characterization of the scattering intensity, average pore size, and fractal dimension evolution of nano-pores in tar-rich coal in different heating rates and temperatures under unconfined pressure conditions. The complementary physical characterization methods such as physical adsorption (BET), thermogravimetric analysis (TG), and scanning electron microscopy (SEM) were also utilized. The research findings reveal the evolution of nano-pore structure characteristics during the underground pyrolysis of tar-rich coal. The research results demonstrate that during the underground pyrolysis process, the average pore size of nano-pores gradually increases with the rise in temperature. Nanopore development is relatively slow during the pyrolysis drying and degassing stage ( < 300 ℃), while the most significant growth occurs during the active stage (300-500 ℃), resulting in a 57.1% increase. Subsequently, nanopore development slows down during the secondary degassing stage. Importantly, it was observed that the low-medium-high temperature (≤800 ℃) pyrolysis conditions does not cause any significant changes in the surface fractal dimension (Ds) of tar-rich coal. Based on SANS spectroscopy analysis, it was discovered that the distribution of nano-pores in tar-rich coal during the pyrolysis process exhibits isotropic characteristics. This suggests that the pyrolysis reaction does not impact the directional development of nanopores in this experimental samples. Furthermore, a comparison between different heating rates (5 ℃/min and 20 ℃/min) reveals minimal influence on the average pore size and fractal dimension of nano-pores during underground pyrolysis of tar-rich coal. Notably, under slower heating rates, tar-rich coal exhibits higher overall scattering intensity, indicating more substantial development of the nano-pore structure. This, in turn, facilitates the progress of the underground pyrolysis reaction. In addition to these findings, the CSNS-SANS technique proves advantageous over the conventional characterization methods such as BET. It enables the detection of closed pores within the samples and provides experimental conditions that closely resemble the in-situ environment, thereby ensuring more reliable results.