To analyze the mesostructure evolution behavior of nitrate ester plasticized polyether （NEPE） propellants under uniaxial quasi-static tensile load， in-situ observation test was conducted on NEPE propellants during the tensile process using Micro CT. The size and an shape of ammonium perchlorate （AP） particles and initial defects in NEPE propellants were characterized. The failure process of the mesostructure in the propellant during uniaxial tensile process was obtained， and the quantitative analysis of the changes in meso-damage of NEPE propellants was carried out using porosity. The reason for the changes of macro-mechanical properties is explained based on the evolution laws of the structure of NEPE propellant on the mesoscale. The results indicate that the initial defects size of NEPE propellant is small and the volume ratio is low， with an average value of 0.12%. In the process of uniaxial quasi-static tension， the meso-failure process of NEPE propellant mainly includes three stages， pores nucleation， growth and convergence. Although the volume fraction of AP particles is low， it is often the start of meso-damage because of easy debonding. HMX particles will also debonding after a certain degree of debonding of AP. The influence of HMX debonding behavior should be considered when analyzing the macro-mechanical properties of NEPE propellant. The nucleation and growth of a large number of mesoscopic defects is the reason why the macro-mechanical properties properties of NEPE propellant enter the nonlinear section. The phenomenon that the increase of macroscopic stress lags behind the increase of strain is more and more obvious because of the continuous convergence of mesoscopic defects. During the loading process， the porosity shows a change trend of slowly increasing first， then sharply increasing and finally increasing tends to be gentle. The change law of porosity can not only quantitatively reflect the evolution stages of the mesoscopic defects of NEPE propellant， but also have a certain corresponding relationship with the changes of the macroscopic mechanical properties of NEPE propellant.