Abstract: With the deepening of exploration and development of oil and gas fields, for some extremely low pressure formation conditions, sometimes low density cement slurry can not meet the needs of safe cementing construction, so ultra-low density cement slurry can only be used to solve the problem of balanced pressure cementing. Due to the fact that the existing low density cementing quality evaluation standards can not objectively and accurately reflect the real bonding quality of ultra-low density cement slurry, the acoustic characteristics and mechanical parameters of ultra-low density cement slurry under different curing conditions are tested, and the factors and rules affecting the mechanical and acoustic characteristics of ultra-low density cement slurry are analyzed. The results show that density, temperature and curing time all have an effect on the development of sound velocity and compressive strength of ultra-low density cement, in which temperature has a greater effect on the early stage and a smaller effect on the later stage. The higher the density of cement, the higher the acoustic impedance and compressive strength, the linear relationship between acoustic impedance and elastic modulus, and exponential relationship with compressive strength. The formula for calculating the relative acoustic amplitude of casing wave of ultra-low density cement cementing is established. The research results provide the basis for formulating the evaluation index of cementing quality of ultra-low density cement slurry.