Abstract: The rheological properties of polymer have profound effects on the displacement efficiency of polymer in reservoir porous medium. This paper investigated the effects of polymer concentration, temperature, metal ions, and mechanical shear on the elongational rheological behavior of ultra-high molecular weight HPAM (U-HPAM) and normal medium molecular weight HPAM (M-HPAM). It was found that the intermolecular entanglement and elongational viscosity increased with HPAM concentration, and U-HPAM exhibited superior ability to enhance the elongational viscosity than M-HPAM. The intermolecular entanglement was reduced by increasing temperature, increasing Na⁺, Ca²⁺, Mg²⁺, Fe³⁺, Fe²⁺ concentrations and enhancing mechanical shear, which correspondingly decreased the elongational viscosity. Temperature and additional Na⁺, Ca²⁺, Mg²⁺, Fe³⁺ had severe impact on the elongational behavior of M-HPAM more than that of U-HPAM, while the additional Fe²⁺ and mechanical shear had the similar effects on the elongational behavior of M-HPAM and U-HPAM. Seepage and core displacement experiments exhibited that U-HPAM had better profile control ability than M-HPAM, and it could play a better role in oil displacement in strong heterogeneous formation.