This paper reports the structure and order degree changes of double perovskite La₂CoMnO₆ induced by Sr²⁺ doping at A-sites (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0), which effectively regulates the electrical and magnetic properties. The evolution of the four-phase structure is from the orthogonal (pbnm, disordered) and monoclinic (P2₁/n, ordered) mixed phases of La₂CoMnO₆ (x = 0) to the rhombohedral (R3c) phase of La_1.6Sr_0.4CoMnO₆, and finally to the cubic phase (Fmm) of LaSrCoMnO₆. All samples exhibit the semiconductor behavior of the three-dimensional variable-range hopping model, and the resistivity decreases first and then increases with increasing Sr content. The low resistivity observed in mixed phases (x = 0.6, 0.8) is primarily attributed to the Co²⁺ to Co³⁺ (carrier) transition, the B-site disordering of low structural symmetry, and the p-n type transition. In ferromagnetic La_2−xSr_xCoMnO₆, Sr doping induces the transition from high-spin Co²⁺ (t_2g⁵e_g²) to high-spin Co³⁺ (t_2g⁴e_g²), as well as the superexchange effect from Co²⁺–O–Mn⁴⁺ to Co³⁺–O–Mn⁴⁺. The introduction of a small amount of the cubic phase (x = 0.6) can enhance the bonding energy of the rhombohedral structure, which inhibits the reversal of the magnetic moment, exhibiting a maximum coercive force (H_c = 7.92 kOe). The discovery of rhombohedral and cubic biphase synergistic effect provides a new idea for the modification design of double perovskite materials.