In order to explore the pore structure characteristics of phase change energy storage backfill and their influence on the strength deterioration of backfill, a composite phase change material was prepared with butyl stearate as the phase change material and expanded perlite as the adsorption medium. Cement and tailings were mixed to prepare backfills with different additive amounts of the composite phase change material. The strength and structure characteristics of the phase change energy storage backfill with different addition amounts were obtained by using the methods of CT (computer tomography) scanning, MRI (magnetic resonance imaging) analysis, and uniaxial compression test, and the influence mechanism was analyzed. The results show that: i) The porosity of phase change energy storage backfill increases gradually with the increase of the addition amount. The macropore porosity increases approximately linearly, the proportion of macropores increases gradually, and the pores approximate to sphere. ii) With the increase of the amount of phase change material, the connectivity of the backfill increases, the pore throat length increases, and the number of macropores increases. The pore throat coordination number is concentrated below 5, and the fractal dimension decreases first and then increases significantly, resulting in complex pore distribution. iii) With 5% additive amount, the uniaxial compressive strength of backfill decreases by 30.2% due to the increase of macropores and pore connectivity. With 10% additive amount, the pore size distribution becomes uniform and the uniaxial compressive strength decreases by 48.9%.