The polycrystalline samples of Pr_0.45(Ca_1−x Sr _x )_0.55MnO₃ (x = 0.0, 0.2, 0.4, 0.6) were prepared by solid-state reaction. Influence of Sr²⁺ substitution for Ca²⁺ at A site on magnetism was studied through the measurements of X-ray diffraction (XRD) spectrum, the magnetization-temperature (M-T) curves and electron spin resonance (ESR) curves. The results show that the magnetism strengthens and charge ordering weakens gradually with the increase of Sr²⁺ substitution amount, respectively. The transition of paramagnetism(PM)-antiferromagnetism(AFM) occurs below the transition temperature of charge ordering (T _CO), and there is little ferromagnetic phase in the antiferromagnetic background at the lower temperature, which results in complex magnetic phases of the samples. Ferromagnetic clusters appear and grow larger with the increase of substitution amount. In addition, the reentrant spin-glass behavior appears at about 41 K. The magnetism phenomenon of the samples is mainly due to the competition between double exchange and Jahn-Teller coupling. The Jahn-Teller coupling becomes weak, and the double exchange wins the advantage in the competition with the increase of substitution amount, which leads to the magnetism variation of the system.