The isothermal section phase diagram of Er-Ni-Si ternary system at 773 K was created by x-ray powder diffraction (XRD) analysis in this paper. Seventeen known ternary compounds ErNiSi, ErNi2Si2, ErNiSi2, Er2Ni3Si5, ErNiSi3, Er3Ni6Si2, ErNi10Si2, Er3NiSi2, ErNi2Si, ErNi5Si3, ErNi4Si, ErNi6Si6, Er33.3Ni18.7−10.7Si48−56 (Er2NiSi3), Er3Ni2Si4, Er33.3Ni8.7−4.7Si58−62 (Er4NiSi7), Er8Ni31Si11, and Er5Ni2Si3 (unknown structure type, similar to Dy5Ni2Si3) have been testified. A new compound Er37Ni3Si60 (unknown structure type) has been confirmed by excluding its XRD pattern as a superposition of nearby phase patterns. Part diffraction peaks have obvious movement with the change of Si content in Er2NiSi3 solid solution, and more moving directions are not consistent, especially at (002) and (110) planes. At the same time, quasi–binary solid solutions were detected in Er2Ni17−xSix, ErNi5−xSix, Er2Ni7−xSix, ErNi3−xSix, ErNi2−xSix, ErNi−xSix, and ErSi−xNix compounds. Other binary compounds of the Er-Ni-Si system do not show any visible solubility of the third element. If the mixture of α-ErSi2 and β-ErSi2 phases is regarded as one ErSi2 phase, the 773 K isothermal section of the phase diagram of Er-Ni-Si system consists of 55 three phase regions and ten solid solution lines.