Strontium ferrites with different Bi2O3 content are prepared by the solid phase method, and their magnetic properties are investigated primarily. The Bi2O3 additive and sintering temperature separately exhibit a strong effect on the sintering density, crystal structure, and magnetic properties of the ferrites. As to the ferrites with 3 wt% Bi2O3, the relatively high sintering density 𝜌s, saturation magnetization 𝑀s, and intrinsic coercivity 𝐻ci can be obtained at a low sintering temperature of 900 ∘ C even much lower. Furthermore, the effective magnetic anisotropy constant 𝐾 eff and magnetic anisotropy field 𝐻a of the ferrites are calculated from the magnetization curve by the law of approach to saturation. It is suggested that the low-temperature sintered SrFe12O19 ferrites with 𝑀s of 285.6 kA/m and 𝐻a of 1564.6 kA/m possess a significant potentiality for applying in the self-biased low-temperature co-fired ceramics circulators from 34 to 40 GHz.