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In this work, a series of compounds in the CaO-CoO_x-ErO_y ternary oxide system were synthesized in air at 885 °C, using a high temperature solid-phase synthesis method. The phase boundary of each solid solution region in the CaO-CoO_x-ErO_y system was determined by X-ray powder diffraction techniques. The phase diagram of the CaO-CoO_x-ErO_y system at 885 °C includes three series of ternary oxide solid solutions: (Ca_3−xEr_x)Co₄O_9−z (0 ≤ x ≤ 0.9), (Ca_3−xEr_x)Co₂O_6−z (0 ≤ x ≤ 1.25), and (Er_1−xCa_x)CoO_3−z (0 ≤ x ≤ 0.33). Four three-phase regions and five solid solution tie-line regions were obtained. The structure of the perovskite solid solution (Ca_3−xEr_x)Co₂O_6−z has been analyzed by Rietveld refinements. With the increase of Er content, the cell parameters of (Ca_3−xEr_x)Co₂O_6−z exhibit a decreasing trend in a and b directions and an increasing trend in c direction. A brief comparison of the phase diagrams of the CaO-CoO_x-RO_y (R = La, Dy, and Er) systems in air at 885 °C is provided.